Short answer
Yes, many were. In the eighth century, Charlemagne's law for the conquered Saxons ordered death for any Saxon who hid to avoid baptism. In 1502 Spain ordered every Muslim in Castile to leave, on terms that made leaving impossible. Baptism was the only way to stay, and a leading historian calls it enforced conversion. In the Americas, Spanish conquerors read out a demand that the people accept the Church as ruler of the world. If they refused, they faced war and slavery. In 1685 France ordered that every enslaved African in its American islands be baptised Catholic. In the Philippines, where Islam had spread by preaching, a Spanish governor in 1578 ordered Muslim preachers seized and the houses where they preached burned. The Qur'an says, "Let there be no compulsion in religion".
Full explanation
The Saxons: baptism or death
Charlemagne, king of the Franks, fought the pagan Saxons for some thirty years. His law for their conquered lands ordered death for burning a church, for eating meat in Lent out of contempt for Christianity, and for burning the dead after the pagan custom. Its eighth chapter went further. Any Saxon who hid to avoid baptism, "and shall have wished to remain a pagan, let him be punished by death".
Charlemagne's law for the conquered Saxons orders death for anyone who burns a church, eats meat in Lent out of contempt for Christianity, or burns the dead after pagan rites. Chapter 8 decrees: if any one of the race of the Saxons hereafter concealed among them shall have wished to hide himself unbaptized, and shall have scorned to come to baptism and shall have wished to remain a pagan, let him be punished by death.
Spain's Muslims: leave, or be baptised
When Spain conquered the Muslim kingdom of Granada, its terms of surrender promised the Muslims their mosques and their own law. Ten years later, a royal decree ordered every Muslim in the kingdoms of Castile and Leon to leave by the end of April. Boys under fourteen and girls under twelve were to stay behind. The conditions made leaving impossible. The exiles could sail only from the far northern ports. They could go neither to the neighbouring kingdoms nor to any Muslim land nearby, on pain of death. The historian Henry Charles Lea explains why the decree named no other choice. It was so that the baptism which alone could save them might be called voluntary. He heads the passage "Enforced conversion in Castile".
Sets out the terms of surrender by which Ferdinand and Isabella received the Muslims of Granada as subjects, promising them their lands, their mosques and their own law. Describes how the forced conversions in Granada followed the arbitrary methods of Francisco Ximenes de Cisneros, Archbishop of Toledo, and how the royal decree (pragmatica) of 12 February 1502 ordered all Moors to leave the kingdoms of Castile and Leon by the end of April, keeping back boys under fourteen and girls under twelve, on conditions that made leaving impossible: they could sail only from ports of Biscay, could not go to Navarre, Aragon, the Turks or the Moors of Africa, and faced death for staying. Lea says the edict offered no stated alternative, so that conversion, the only way to escape it, could be called voluntary; he heads the passage "Enforced conversion in Castile".
The Americas: submit to the Church, or face war
Spanish conquerors read out a document called the Requerimiento, the Requirement, to the peoples they met. It told them to accept the Church as ruler of the whole world. They were to accept the Pope, and in his name the king and queen of Spain, as their lords, and to let priests preach to them. It said they would not be forced to become Christians. But if they refused or delayed, it declared: "we shall powerfully enter into your country, and shall make war against you". They, their wives and their children would be taken as slaves.
A demand read out by Spanish conquerors to the peoples of the Americas. Resting on the Pope's donation of their lands to the kings of Spain, it required them to acknowledge the Church as ruler of the whole world, the Pope, and in his name the king and queen of Spain as their lords, and to let priests preach to them. It said they would not be compelled to turn Christians. But if they refused or delayed, it declared: "we shall powerfully enter into your country, and shall make war against you in all ways", take you, your wives and your children and make slaves of them, and say the deaths and losses that follow are your fault.
Enslaved Africans: baptised by law
In 1685 Louis XIV of France issued the Code Noir, the law for slavery in France's American islands. Its second article orders that all slaves in the islands be baptised and instructed in the Catholic religion. Settlers who bought Africans newly landed from the slave ships had to report them within a week, so that they could be baptised. The enslaved were not asked.
The French royal edict governing slavery in France's American islands. Its second article orders that all slaves in the islands be baptised and instructed in the Catholic, Apostolic and Roman religion, and that settlers who buy newly arrived Africans report them within a week to the governor and intendant, who are to have them instructed and baptised.
The Philippines: Islam by preaching, then war against it
Islam reached the southern Philippines by preaching, not conquest. Najeeb Saleeby wrote a history published in Manila in 1908. He tells of a preacher called Makdum who reached Sulu about 1380 and made converts. He tells of Abu Bakr, who came about 1450, married into the ruling family and taught religion and law. Saleeby says the change was slow and gradual, but real. It corrects, he says, the old belief that Islam came there by the sword. Spain arrived in the 1560s. In 1578 Governor Francisco de Sande sent an armed expedition to Sulu. His orders were to forbid any more preachers "of the doctrine of Mohammed", to "seize and bring them before me", and to "burn or destroy the house where that accursed doctrine has been preached". Saleeby lists among Spain's aims "to uproot the 'accursed doctrine' of Mohammed and to convert the Sulus to the Christian religion". Saleeby says the expedition began a long state of war between Sulu and Spain.
Says Islam reached Sulu through preachers: Makdum, who arrived about 1380 and made converts in many places, and Abu Bakr, who arrived about 1450, married the daughter of Raja Baginda and taught religion and law, so that the people abandoned their former gods in a slow and gradual but real conversion; Saleeby says this modifies the opinion formerly held that Islam was introduced there by the sword. It gives Governor Francisco de Sande's instructions of 1578 for the expedition against Sulu: to order the Sulus not to admit any more preachers of the doctrine of Mohammed; to find out who preached it, and "seize and bring them before me"; and to burn or destroy the house where that accursed doctrine has been preached. Saleeby lists among the aims to reduce Sulu to a vassal state, to exact tribute, and "to uproot the 'accursed doctrine' of Mohammed and to convert the Sulus to the Christian religion", and says the expedition began a state of war between Sulu and Spain.
No compulsion in religion
The Qur'an forbids forcing faith on anyone: "Let there be no compulsion in religion: Truth stands out clear from Error". It asks the Prophet (ﷺ), "wilt thou then compel mankind, against their will, to believe!" It tells him that his task is only to remind, not to control people. A faith forced on someone is not faith at all, and the Qur'an leaves each person to choose.
“Let there be no compulsion in religion: Truth stands out clear from Error: whoever rejects evil and believes in Allah hath grasped the most trustworthy hand-hold, that never breaks. And Allah heareth and knoweth all things.”
“If it had been thy Lord's will, they would all have believed,- all who are on earth! wilt thou then compel mankind, against their will, to believe!”
“Therefore do thou give admonition, for thou art one to admonish. Thou art not one to manage (men's) affairs.”
“Say, "The truth is from your Lord": Let him who will believe, and let him who will, reject (it): for the wrong-doers We have prepared a Fire whose (smoke and flames), like the walls and roof of a tent, will hem them in: if they implore relief they will be granted water like melted brass, that will scald their faces, how dreadful the drink! How uncomfortable a couch to recline on!”
Perspectives
Christian
Many Christians say forced conversions were the crimes of kings and soldiers, not the teaching of Christ or the Church. They say true faith cannot be forced.
Forced conversions were the work of rulers, not of the Church.
Rebuttal
The Requerimiento demanded that the conquered accept the Church as ruler of the whole world. They were also to accept the Pope's grant of their lands to Spain. In Spain, Lea traces the forced conversions of Granada to the arbitrary methods of Ximenes, the Archbishop of Toledo.
The Requerimiento itself said no one would be forced to become a Christian.
Rebuttal
It did say so. Then it threatened war, slavery and the taking of wives and children from anyone who would not accept the Church as the world's ruler and let its priests preach. A choice between that and submission is no free choice.
The Islamic perspective
Christian rulers forced baptism on the Saxons, on Spain's Muslims and on enslaved Africans. They conquered the Americas in the Church's name. They made war on the Muslims of the Philippines to uproot their faith. The Qur'an forbids compulsion in religion.
Charlemagne's law ordered death for any Saxon who hid to avoid baptism.
Spain's decree of 1502 left Castile's Muslims no way to stay but baptism.
France's Code Noir of 1685 ordered every enslaved person in its American islands to be baptised Catholic.
Islam reached the Philippines by preaching. In 1578 a Spanish governor ordered its preachers seized, and their places of preaching burned.
The Qur'an says there is no compulsion in religion.