Under Abu Bakr, Islam only regained ground lost in the dissension that occurred among the tribes that had united with Mohammed during the Prophet's life.
Islam did spread considerably during the reigns of the subsequent Caliphs due to the conquests of Islamic Armies. 'Omar, 'Othman, 'Ali, Mu'awiya, and Yazid I all expanded the reach of Islam through successful attacks on non-Muslim States or regions.
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The role of Islamic armies was just to free the will of oppressed people and to leave them to select or maintain the faith that they believe in freely. Islam spread mainly due to its logic beliefs, authenticity of Quran, good morals, and honesty. refer to questions below for more details.
The Sahara Desert stood as a powerful border for Muslim armies for several centuries.
Saladin
711 - 712 AD
The Muslim culture changed Cordoba by the Muslims treaties, mercenaries, and standing armies.
Magyars, Muslim armies, and Vikings
They let people keep there own religions
they conquered all except the north
Leadership and Physical courage
spain
Saladin led the Muslim armies in their opposition to the Christian Crusaders.
Battle of Tours,France .
Because they let people they conquered keep their religion.