The Crusades, wars by European Christians against the Muslims world are sometimes confused with early Islamic Conquests of 7th Century CE, when Muslims fought against their contemporary rivals including Christian Byzantine Empire. Unfortunately, some even ignorantly claim "Muslims attacked Christians first" and that resulted in Crusades of 11 Century CE. Despite the fact that the Papal Authority of 11th Century did use the Islamic Conquests as a pretext to launch Crusades yet there is a huge difference between the Crusades and the wars Muslim armies fought against their regional rivals. As per dictionary.com the term "Crusades" is defined as "any of the military expeditions undertaken by the Christians of Europe in the 11th, 12th, and 13th centuries for the recovery of the Holy Land from the Muslims." The Muslim attacks against Byzantine Christian Empire was not based on religious zeal to fight Christian faith where as Crusades were particularly waged with a mindset to eliminate Islam and Muslims especially from the Holy Land and its precincts. The following three major differences between Crusades and Early Islamic Conquests discussed further will clarify this topic.
Firstly the crusades were specifically aimed at getting rid of Islam and Muslims from the Holy Land of Jerusalem and its adjoining areas while Muslim armies attacking Christian Byzantine empire did not do so with this aim. The main reason that Muslim armies battled Byzantine Empire was because the former's emergence as a new regional power brought them face to face in a war situation with 2 hostile regional neighbors: Sassanian Empire and the Byzantine Empire. Before Islam these 2 main regional powers were already at war with each other for centuries for the control of entire region, please read Roman–Persian Wars. After the acceptance of Islam the Arabs gained strength in the Arabian Peninsula and sooner or later the armies of these 2 empires that could not stand each other were naturally not going to accept a third upcoming power in their neighborhood. In fact the showdown of Muslims with these 2 empires was already inevitable when they displayed hostility to Prophet Muhammad's emissaries. Khusrow Pervez - Khusrow II (Chosroes II) the then Sassanian Ruler after receiving the Prophet's letter "tore the letter into shreds and forthwith dictated a command to his viceroy in Yemen to send a couple of troopers to arrest the Prophet and bring him to his presence." The first conflict between Muslims and Byzantine empire happened in Battle of Mu'tah. This happened as a result of an emissary that Prophet Muhammad ﷺ sent to ruler of Bosra, in modern day Syria, and on his way, he was intercepted and killed by al-Ghassani, a Christian Arab governor and a close ally to the Byzantine Emperor Heraclius (ruled 610 to 641 CE). Killing a peace emissary was considered an act of war. This led towards the famous Expedition of Tabuk led by Prophet Muhammad ﷺ himself to show a force of strength to the Byzantines. Such incidents led the Muslims towards wars with their hostile neighbors that included not only Christians but also the fire worshiping Magians of Sassanian empire.
Secondly and most importantly for many Christians who may not know, especially in the mainstream West, that when Arab Muslim armies were fighting Byzantine, the Christians of the region in many cases sided with the Muslims due to obvious reasons that Muslims were not waging a sort of a Crusade against people of other faiths. They were in fact seen as a liberating force against the Byzantine and Persian oppression. This can be confirmed from the words of the Christian Priests and religious leaders of early Islamic history. T. W. Arnold writes in his Preaching of Islam:
"Michael the Elder, Jacobite Patriarch of Antioch, writing in the latter half of the twelfth century, could approve the decision of his co-religionists and see the finger of God in the Arab conquests even after the Eastern churches had had experience of five centuries of Muhammadan rule. After recounting the persecutions of Heraclius, he writes: "This is why the God of vengeance, who alone is all-powerful, and changes the empire of mortals as He will, giving it to whomsoever He will, and uplifting the humble — beholding the wickedness of the Romans who, throughout their dominions, cruelly plundered our churches and our monasteries and condemned us without pity — brought from the region of the south the sons of Ishmael, to deliver us through them from the hands of the Romans. And, if in truth, we have suffered some loss, because the catholic churches, that had been taken away from us and given to the Chalcedonians, remained in their possession; for when the cities submitted to the Arabs, they assigned to each denomination the churches which they found it to be in possession of (and at that time the great church of Emessa and that of Harran had been taken away from us); nevertheless it was no slight advantage for us to be delivered from the cruelty of the Romans, their wickedness, their wrath and cruel zeal against us, and to find ourselves at peace."
T. W. Arnold mentions another incident when Muslim armies under Abu Ubayda (r.a), a companion of Prophet Muhammad ﷺ, reached the valleys of Jordan and took control of the area, the Christians welcome them with open arms due to the kind treatment of their non-Muslim subjects for which they were well known. The writer says: “When the Muslim army reached the valley of the Jordan and Abu Ubaydah pitched his camp at Fiḥ l, the Christian inhabitants of the country wrote to the Arabs, saying: "O Muslims, we prefer you to the Byzantines, though they are of our own faith, because you keep better faith with us and are more merciful to us and refrain from doing us injustice and your rule over us is better than theirs, for they have robbed us of our goods and our homes." The people of Emessa closed the gates of their city against the army of Heraclius and told the Muslims that they preferred their government and justice to the injustice and oppression of the Greeks.”
In many cases Arab Christian tribes sided with the Muslim armies, such as Banu Taghlib, assisted Muslims in their battles against Persian armies and retained their Christian identity under the Muslim rule for centuries after. The Christian tribe of al-Jurajima that lived near Antioch sided with the Muslim armies against Byzantine forces and so did the Coptic Christians of Egypt who preferred to be ruled by Muslims than their coreligionists. This attitude of the Muslim rulers towards their subjects was unparalleled in history. The Muslim attitude of war was not a holy war even though the attitude of those who were fighting was holy in mind i.e. fighting against oppression, but the wars itself were never intolerant religious conflicts, as still portrayed in the Western minds, for had they been like Crusades, Muslim armies would not have been tolerant towards their non-Muslim subjects. This is because intolerance is something which clearly contradicts the teachings of Quran which the early Muslims were very strong in following.
Thirdly when doing a comparison, Muslim wars against Byzantine and the Crusades against Muslims are 2 completely opposite things. The crusades were specifically waged with the concept that Muslims and Islam was a scourge and it needs to be eliminated. This propaganda, of which the primary source was the Pope himself, was proven by the treatment of the Muslims by their European conquerors. Mass murders and rape of Muslims was their attitude, they considered these acts justified according to the interpretations taught by their church. On the other hand Muslims were guided by principles to respect other people’s faiths and honor their lives and properties as long they are not rebellious. A simple comparison of the Muslims when they entered Jerusalem, with that of Crusaders clarifies this for us. When Jerusalem submitted to the Prophet's 2nd successor and Caliph, Umar ibn al-Khattab (r.a) he made the following agreement with the Christian Patriarch Sophronius of the city:
"In the name of God, the Merciful, the Compassionate. This is the assurance of safety which the servant of God, ʿUmar, the Commander of the Faithful, has given to the people of Jerusalem. He has given them an assurance of safety for themselves, for their property, their churches, their crosses, the sick and healthy of the city and for all the rituals which belong to their religion. Their churches will not be inhabited by Muslims and will not be destroyed. Neither they, nor the land on which they stand, nor their cross, nor their property will be damaged. They will not be forcibly converted. And [as per the Christians’ request] Jews will not live in the same areas as them."
The following are short accounts of how the Crusading Kings of Europe when entered the same holy city slaughtered its inhabitants. The following chroniclers of the First crusade recorded their accounts of the incidents as mentioned in "The Siege and Capture of Jerusalem: Collected Accounts". According to the Gesta Francorum, speaking only of the Temple Mount area, "...[our men] were killing and slaying even to the Temple of Solomon, where the slaughter was so great that our men waded in blood up to their ankles..." According to Raymond of Aguilers, also writing solely of the Temple Mount area, "in the Temple and porch of Solomon men rode in blood up to their knees and bridle reins." Writing about the Temple Mount area alone Fulcher of Chartres, who was not an eyewitness to the Jerusalem siege because he had stayed with Baldwin in Edessa at the time, says: "In this temple 10,000 were killed. Indeed, if you had been there you would have seen our feet coloured to our ankles with the blood of the slain. But what more shall I relate? None of them were left alive; neither women nor children were spared."
Much of the reason why Muslims were welcomed by the Christian residents of the lands they conquered were because their Byzantine rulers belonged to the Orthodox Church which displayed intolerance towards Christians of other thoughts and persecuted them. The Muslims gave them the opportunity to practice their Christian faith as they wished without restrictions. T. W. Arnold writes further how the Coptic Christians of Egypt welcomed the Muslim army under Amr ibn Al Aas (r.a), a companion of Prophet Muhammad ﷺ:
“The Jacobites, who formed the majority of the Christian population, had been very roughly handled by the Orthodox adherents of the court and subjected to indignities that have not been forgotten by their children even to the present day. Some were tortured and then thrown into the sea; many followed their Patriarch into exile to escape from the hands of their persecutors, while a large number disguised their real opinions under a pretended acceptance of the Council of Chalcedon. To these Copts, as the Jacobite Christians of Egypt are called, the Muhammadan conquest brought a freedom of religious life such as they had not enjoyed for a century. On payment of the tribute, 'Amr left them in undisturbed possession of their churches and guaranteed to them autonomy in all ecclesiastical matters, thus delivering them from the continual interference that had been so grievous a burden under the previous rule; he laid his hands on none of the property of the churches and committed no act of spoliation or pillage.”
History bears witness to the fact that Muslim expansion in the earlier years of Islam were not due to prejudice, hate, racism based on a religious pretext. Yes Muslims did attack their opponents and defeated them and build an empire on their lands but provided justice which the people of that land were deprived of. These people were yearning for someone to come save them from the persecution they faced at the hands of the authorities ruling over them. Therefore even though the Islamic Conquests of 7th Century CE happened first, but the factors guiding their conquests were not intolerance and hatred like that of Crusaders.
 al-Mubarakpuri, Safi-ur-Rahman. 1996. Al-Raheeq Al-Makhtum (The Sealed Nectar). Dar-us-Salam Publications.
 THE PREACHING OF ISLAM A History of the Propagation of the Muslim Faith - Pg 47 Chapter III - The Spread of Islam Among the Christian Nations of Western Asia
 THE PREACHING OF ISLAM A History of the Propagation of the Muslim Faith - Pg 48 Chapter III - The Spread of Islam Among the Christian Nations of Western Asia
 Recorded in Tarikh Al Tabari, - https://www.islam21c.com/islamic-thought/the-treaty-of-umar/#_ftn2 - also quoted in THE PREACHING OF ISLAM A History of the Propagation of the Muslim Faith - pg 49 Chapter II.
 Medieval Sourcebook: The Siege and Capture of Jerusalem: Collected Accounts - History Department of Fordham University, New York https://sourcebooks.fordham.edu/source/cde-jlem.asp#fulcher1
 THE PREACHING OF ISLAM A History of the Propagation of the Muslim Faith - pg 83 - Chapter IV - The Spread Of Islam Among The Christian Nations Of Africa
The Conquest of Constantinople (1453 CE) & Conversion of Hagia Sophia into Masjid - An Islamic Viewpoint
Originally written on July 18, 2020 on Quora - Did Prophet Muhammad (saw) predict the Conquest of Constantinople (1453 CE) and as per Islam, was it justified to convert Hagia Sophia Church into a Masjid?
Turkish President Recep Tayyip Erdogan on Friday (July 10th) formally re-converted Istanbul’s sixth-century iconic Hagia Sophia into a mosque, hours after a high court annulled a 1934 decision that had turned it into a museum. For many around the world who are not much familiar with the background of the conversion of Hagia Sophia church in to masjid by 15th century conqueror Sultan Mehmed II Al Fateh (the Conqueror), are questioning this decision of the Turkish government. Apparently it is forbidden for Muslims to take over and forcibly convert non-Muslim places of worship in to masjid but this historic decision was exclusive, it was necessary for that time and situation.
The Prophet’s Prediction of Constantinople
The prediction of the conquest of Constantinople and Rome was given by Prophet Muhammad ﷺ in several traditions, long before him and the earlier Muslims hardly exercised any control in the Arabian Peninsula. In fact, their influence was only limited to a small town called Yathrib (later became Madinah). As per Islamic scholars the first conquest of the city is implied recorded in the following reliable tradition:
Abu Qateel said, "We were with Abdullah ibn Umar (r.a), and he was asked, “Which city will be conquered - Constantinople or Rome?” 'Abdullah (r.a) called for a box to be brought, and he removed from it a book. Abdullah (r.a) said, “While we were (gathered) around the Messenger of Allah ﷺ and we were writing, the Messenger of Allah ﷺ was asked: ‘Which city will be conquered first - Constantinople or Rome?’ The Messenger of Allah ﷺ said: ‘The city of Hiraql (Heraclius) will be conquered first’ - i.e. Constantinople."' (Recorded in Musnad Ahmad and in Al Bidayah Wa Al Nihaya by Imam ibn Kathir - The Prophets Intimated That The Muslims Would Conquer Constantinople Before Rome - pg 71)
At this point it is also important to mention that there are other traditions that speak about a second Conquest of Constantinople by Muslims that would take place in future because Muslims will end up losing their control of this city to the Romans (most likely modern day Europeans). However, we are only concerned with the one mentioned above.
Ibn Khaldun, 14th century Arab Islamic scholar, social scientist and historian also called the father of the modern disciplines of historiography, sociology, stressed in his masterpiece Al Muqaddimah that a dynasty is like the center of a light and its rays or the water circles made when the water is struck in the center, it is strong near the center but becomes weaker as it expands. The dynasty remains strong as long as its center or capital remains strong but gradually begins to crumble at its extremities. The decay starts from the extremities, and if allowed to gradually reach the center the dynasty ceases to function and exist. The center is like the heart from which the (vital) spirit reaches the entire body, if it stops functioning, the entire body stops. He gives the example of Persian Sassanid Empire that was dissolved immediately because their capital Al Madain (Ctesiphon) was overrun in the early years of Islamic conquests, as opposed to Byzantine empire that lost all of its territories in Greater Syria yet continued to exist as long as their capital Constantinople was under its authroity. The loss of the capital is the end of an empire and this is what is meant by the following Ayah (verse) in the Quran:
(Al Quran 44:25-29) how many gardens did they leave behind, and water-runnels, and fields of grain, and noble dwellings, and [all that] life of ease in which they used to delight! Thus, it was. And [then] We made another people heirs [to what they had left], and neither sky nor earth shed tears over them, nor were they allowed a respite.
The following discussion explains how the gradual decaying of the Byzantine empire led towards their defeat at the hands of the Ottomans and complete loss of all its power at the eventual loss of their capital.
The Byzantine Empire During Middle (9th Century CE) & Late Period (14th Century)
After the loss of all Byzantine states in the Middle East at the hands of the early Islamic conquests in the mid-7th century and religious schism between Rome (Catholic Church) and Constantinople (Orthodox Church) in 1054 CE, the Byzantine Empire was a medium sized regional state based in Constantinople and fighting a battle for survival. During the 11th Century, not only was it under the threat of the Muslim Turkish tribes from its Eastern Front but also their Catholic co-religionists who attacked during the Fourth crusade in 1204 CE, and ruled Constantinople till its recapture in 1261. But the retaking of the capital by the Byzantines and its attempts to revival were not very successful because the empire had already lost much of its influence and power that it had before the loss of its capital to the Latin Christians. After recapturing Constantinople, the Byzantines continued to struggle against the crusades of the Catholic church and their attempts to retake the capital. Meanwhile the Eastern border lands under the control of various Turkish tribes started strengthening themselves and in particular one of them under the leadership of Osman 1 (from where comes the term Ottoman or Osmanli) began capturing lands and established themselves as independent rulers. As Byzantine empire shrank in size and became weaker in its political power in the region, they also suffered economically. The increase in the refugees fleeing from territories lost to their rivals and the decrease of treasury wealth caused the Byzantine heavy economic loss. The Byzantine power relied on outsiders for assistance against their rivals such as their support on the Catalan Company – an army of Catalonian mercenaries – to protect their territories as well as their reliance on the Mongols to fight the rulers of Turkish states (beyliks) caused them to become weaker and more dependent on others for their political power. The weak military, political and economic Byzantine kingdom could not protect nor support their own subjects from the occupation of others nor fight the corruption of their own. Additionally, the struggle for throne among contenders and the Byzantine civil war (1341-1347 CE) caused the Kingdom to overlook the challenges from outside. All these events and the failure of Byzantine revival, proved that it was time for the weak empire to be replaced by a powerful one who would not only replace them but also have the power to establish justice and order that would provide safety and protection which the subjects of the empire were deprived of.
Byzantine Relationship with Turks
The Byzantines had a continuous relationship of war and peace with the Turks from the time of Seljuk Sultanate (empire) who entered the land of Anatolia (modern day Turkey) during 9th century, with the support of several Turk nomad tribes of Central Asia. The famous Battle of Manzikert (1071 CE) where a large Byzantine army suffered defeats at the hands of much smaller Seljuk Turk force, was the beginning point in history for Turks to establish themselves as permanent residents in the former Byzantine Eastern lands of Anatolia. After the Seljuk state became weak by Mongol invasion, eventually becoming a vassal state, new Turk nomad tribes that moved into Anatolia from Central Asia escaping Mongol attacks, among them the Osmanlis (Ottomans) began establishing themselves on the crumbling Seljuk state. With the passage of time they reached such strength that Byzantine Emperor John VI Kantakouzenos (1347 – 1354 CE) was assisted by the Ottoman Sultan Orhan to gain his throne in Constantinople during the Byzantine civil war. John also married his daughter Princess Theodora in return for this favor. But soon the next emperor John V Palaiologos (r. 1354–91), who succeeded Katakouzenos, looked forward to another crusade by the Roman Pope against the Ottomans. The famous Union of Churches decided in Council of Ferrara-Florence (1438–39 CE) to launch a long-awaited crusade which was fully supported and blessed by Eastern Roman Emperor, the Patriarch of Constantinople and representatives of several Orthodox churches. However, Ottoman Sultan Murad I defeated the crusaders at Varna in 1444 and at Kosovo in 1448.
Fall of Byzantine Capital - Constantinople
After Mehmed II, Al Fateh (the Conqueror), became an Ottoman Sultan, the Byzantine emperor suggested that a grandson of the late Prince Suleyman, called Orhan, who lived in exile at Constantinople was a pretender to the Ottoman throne. His unnecessary intervention in to Ottoman ruling affair was a failure. Mehmed II already started planning the siege and the eventual takeover of the historic and legendary city. On July 1st, 1453 CE the Ottoman army entered Constantinople, the jewel in the Byzantine crown was announced as the capital of the expanding Ottoman state. Additionally, the prestige and power of the Orthodox Church, the center of which was Hagia Sophia, was tied directly to that of the Byzantine Empire. As the empire’s political power shrank in the preceding years, so too did the ability of the patriarch, the spiritual head of the Church, to exercise religious authority over Orthodox faithful, and this resulted in divisions among the Orthodox denomination. After the conquest Sultan Mehmed II, Al Fateh (the Conqueror) declared himself the protector of the Orthodox Church, appointed Gennadios as the Patriarch, the spiritual head of the Orthodox Church. The Patriarch, centered in Constantinople, was not only declared as the official spiritual head of all Orthodox faithful but in fact was able to consolidate and solidify his religious authority over all Orthodox in the former Byzantine territories, as Ottomans established their rule over those territories. In fact the Orthodox population in the former Byzantine territories, numerically superior to the Turks, accepted the rule of the Sultan over the failed and corrupt ruling system of the Byzantine authorities and the tyrannical rule of Catholic Franks and Venetians that ruled in certain parts of the empire under a feudal system that had reduced the population to serfdom. The Christians under Ottomans were fully permitted to practice their own law in all civil matters under the authority of their church with no interference from Muslim authorities whatsoever. Professor T. W. Arnold quoting Russian annalists (experts on Roman history) who speak of the fall of Constantinople due to same reasons: "Without the fear of the law an empire is like a steed without reins. Constantine and his ancestors allowed their grandees to oppress the people; there was no more justice in their law courts; no more courage in their hearts; the judges amassed treasures from the tears and blood of the innocent; the Greek soldiers were proud only of the magnificence of their dress; the citizens did not blush at being traitors; the soldiers were not ashamed to fly. At length the Lord poured out His thunder on these unworthy rulers, and raised up Muhammad, whose warriors delight in battle, and whose judges do not betray their trust.” (CHAPTER VI. THE SPREAD OF ISLAM AMONG THE CHRISTIAN NATIONS IN EUROPE UNDER THE TURKS - Preaching of Islam, pg 115 - T. W. Arnold) Hence the gradual disintegration of the Byzantine empire due to its oppressive rule of the kings and corrupt government officials caused Qudrat (the Divine) to replace them with people who were just in their rule and provided security to the lives and property of their subjects.
The legality of converting Hagia Sophia into Masjid
In Byzantine political ideology, as long as the empire retained its three elements—the emperor, the patriarch at Hagia Sophia Church, and the city (Constantinople)—it continued to exist. After the decision to make this city into the future capital of the Ottoman state, Mehmed II Al Fateh (the Conqueror) decided to convert Hagia Sophia (Church of Holy Wisdom), the ancient church that played a political role in the establishment of the Byzantine empire, in to a masjid, a Muslim place of worship. Hagia Sophia, a state Church, was unlike other Orthodox churches throughout the Byzantine lands. It played a very active political role in the establishment of the empire. As per their tradition Emperor Justinian (527 to 565 CE) received the plan of the church from an archangel. It was the seat where all Emperors were crowned and blessed by the Church before resuming the throne. The capital of the new Islamic State could not have function with its Christian political legacy. The Islamization of Constantinople was necessary for it to function as a capital of the Ottoman state, hence its name was changed to Islami bol (i.e. plenty Islam in it!) or Islambol, which continued through 18th century. Since the Byzantine empire became part of the museum of history, all the sections of its society that particularly played a political role in sustaining it were also going to be part of the museum, including the main Church. After conversion of Hagia Sophia in to Aya Sophia Cammi (Masjid) the Orthodox patriarchate was moved to the Church of the Holy Apostles, another 4th Century structure in Constantinople, which became the city's main Orthodox cathedral.
Some Muslims, and even non-Muslims, dispute the fact of converting this in to a Muslim place of worship was a great sin and in support of their argument they claim that Prophet Muhammad’s successor, Caliph Umar ibn Al Khattab (r.a), after entering Jerusalem rejected the request to offer prayer at the famous Church of Holy Sepulcher, in case future generations will follow his footsteps. What they need to realize is that Holy Church in Jerusalem was a general holy site, mainly religious in its functioning, and did not exercise any political influence or authority in ruling policy of the state such as those by capital of Catholic Church in Rome or Orthodox Church in Constantinople. The support for crusades and other wars fought by the Byzantine Emperors or other European Christian rulers were fully sanctioned and backed by these churches while the Church of Holy Sepulcher in Jerusalem, has never played any direct role in the history of politics. What Islam explicitly forbids (Al Quran 22:39-40) is attacking and conversion of those non-Muslim religious sites that are purely used for the purpose of worship and religious activities.
Turkey's President Formally Re-Converts Hagia Sophia Into a Mosque - Time.com
Encyclopedia of the OTTOMAN empire - Gabor Agoston Georgetown University, Washington, D.C. Bruce Masters Wesleyan University, Connecticut
Al Bidayah Wa Al Nihaya (The Beginning of the End) by Imam ibn Kathir
CHAPTER VI. THE SPREAD OF ISLAM AMONG THE CHRISTIAN NATIONS IN EUROPE UNDER THE TURKS - Preaching of Islam, pg 115 - T. W. Arnold
Narrative of Travels in Europe, Asia & Africa by Evliya Effendi
Al MUQADDIMAH - Abd Ar Rahman bin Muhammed ibn Khaldun - Translated by Franz Rosenthal
Church of the Holy Sepulchre - Wikipedia.com