Trade was far more important to the spread of Islam than war. Muhammad was himself a merchant, and because of this merchants and trade are highly regarded in Islam. The Koran states that
the honest merchant will stand with the righteous on the day of judgment.
Islamic merchants carried their religion with them. Those with whom they traded found it convenient to adopt the Islamic faith to facilitate trade. Additionally, the teachings of Islam that all are equal in the eyes of God, and there was therefore no class distinction, made it attractive to many, particularly Hindus in India who suffered under the caste system.
However, war was an important element. Islam divides the entire world into the House of Islam and the House of War, the latter being that portion of the world that has not yet converted. The early spread of Islam was by military means, into Persia, and northern Africa. In 1453, Islamic forces captured the Byzantine capital of Constantinople, now known as Istanbul. Even so, Christians and Jews were not forced to convert by Islamic armies. They were considered "people of the book" who deserved protection, even though they were forced to pay a tax.