Let's start with the prophet Muhammad (PBUH) and look at his example. He did preach Islam peacefully in Mecca for 13 years without once raising a finger against his enemies even though he and his followers were harshly persecuted, tortured, mocked and sometimes murdered. The first wars with Mecca did not occur until after the hijra to Medina, and those battles were defensive against Abu Sufyan and his amassed troops. After the Treaty of Hudaybiyya, the peace treaty between the Muslims and the Meccans, was unilaterally broken by the Meccans, the Muslims conquered Mecca without any bloodshed. Tribal customs would dictate that the Meccans should then be slaughtered for their transgressions against the victorious Muslims, and indeed the Meccans anticipated this, but Muhammad (PBUH) granted them amnesty on the condition that they cease fighting the Muslims. This was a beautiful act of mercy which led to the majority of Meccans choosing to convert to Islam, though no person was ever forced to do so.
What is conversion to Islam? Converting to Islam is something that has to be done by choice, it is something that you believe in your heart, that there is only one God and that Muhammad is His prophet and messenger. Words without belief are meaningless. Of course God alone knows what is in each and every person's heart, so you won't be able to fool God by meaningless lip service to Islam, and religion is solely for God and the benefit of the individual. So, forced conversion is impossible. The Qur'an attests to this fact in 2:256 where it states: "There is no compulsion in religion."
So how did Islam spread so rapidly across North Africa to the Iberian Peninsula and east across India and Central Asia all the way to Southeast Asia? Primarily it spread by trade. Very often while Muslim merchants were traveling abroad they would impress the people they did business with by their honesty and integrity, so much so that those people would ask questions, learn about Islam, and eventually choose to convert.
However, there were indeed wars as well, most of which were in the times of the first four caliphs. The prophet Muhammad advised his armies of how to approach the non-Muslim governments, he told his armies to call them first to Islam, and if they accept it then leave them alone. If they reject Islam, call on them to submit to the Muslim government and pay the jizyah tax, a poll tax paid by non-Muslims to the Muslim government for their protection and upkeep of the lands and buildings. If they refused both of those options, then they were to fight. But, the fighting itself had many rules, it wasn't just a bloodbath, free-for-all like we've seen in other times, for instance by the Mongols and the Christian Crusaders, rather it was warfare waged only against the armies themselves. It was forbidden to fight civilians and it was forbidden to destroy their agriculture and means of livelihood. There was no raping, pillaging and plundering by the Muslim armies; they fought only the armies sent out by the government until they surrendered and left the innocent civilians alone.
There are numerous hadiths of the prophet (PBUH) where he clearly distinguishes between the combatants and the non-combatants. In a hadith related by Abu Dawud he said, "Do no kill any old person, any child, or any woman," and he said, "Do no kill people sitting in places of worship." Abu Bakr, the first Caliph, gave detailed instructions to his army heading for Syria based on his knowledge of Qur'an and hadiths:
"Stop, O people, that I may give you ten rules for your guidance in the battlefield. Do not commit treachery or deviate from the right path. You must not mutilate dead bodies. Neither kill a child, nor a woman, nor an aged man. Bring no harm to the trees, nor burn them with fire, especially those which are fruitful. Slay not any of the enemy's flock, save for your food. You are likely to pass by people who have devoted their lives to monastic services; leave them alone."
Once a government had fallen into the hands of the Muslims, the citizens paid a poll tax and were free to practice any religion they chose. Christianity and Judaism flourished under Islamic rule since the Muslims protected their churches and synagogues and other holy places, even though they had the power to wipe them out if they had so chosen. Perhaps the most amazing law of all is that if a Christian or a Jew committed a crime and was found guilty, they were tried by their own courts and punished by their own laws, not by the courts and laws of the Muslims. Can you imagine in America today a Muslim being tried in an Islamic court rather than the American system? This was truly progressive governing.
Many of the citizens welcomed the Muslims as their rulers and viewed them as liberators who had rescued them from oppressive kingships, granting them their basic human rights and so much more. Many of the citizens of a conquered land would choose to embrace Islam once they came in contact with the Muslims, and Islam did spread rapidly through these acquired lands. But, individuals were never forced to convert to Islam.
The proof that individuals were not and are not forced to convert to Islam is in the facts of populations. In Egypt there are over 14 million Coptic Christians, in Syria, Jordan, Lebanon, Turkey and all the other lands ruled by Muslims there have continuously been large populations of Christians and Jews. Muslims ruled over Spain for more than 800 years, yet the Christian and Jewish populations have thrived throughout that time. There are over 450 million Muslims in Indonesia today but there was no war by Muslims that brought the religion there, it was only merchants. And, perhaps the most fascinating, the fastest growing religion in the United States and Europe is Islam. Are Americans and Europeans being forced to convert? Of course not, it's something else. These people see truth and justice in Islam that they don't find in any other religion or way of life. These people choose Islam for themselves, by themselves, just like the Meccans did 1400 years ago. That is the truth about how Islam has spread throughout the world.