What factors led to the rapid growth of Islam following Muhammad's death, despite early internal conflicts between Sunni and Shia?

Even though there were divisions in the Muslim world after Muhammad's death, the Islam still continued to spread out and became dominant in lot of new territories. There were two big factors as to why this happened. One of them was that all of the caliphates had a very powerful military, so they were able to have successful military campaigns and conquer new territories. Another big factor was that the majority of the countries/empires that were surrounding the caliphates were in the their best conditions, being on the downfall, and being pretty weak, which just made it much easier for the Islam to spread out.