Book is mightier than army
Knowledge is power. Pen is mightier than a sword, and book is mightier than a king with army.
Imagine medieval European kings and the clergy. The clergy had book and knowledge although those books (i.e., the Bible) were deviated, but still they preserved some divine tradition and history. However, since the kings had no knowledge, they always had to be under mercy of the church (in medieval Europe).
Was not that amazing that the armed forces was on the side of the kings, and yet the Church was victorious. Indeed, it is amazing that knowledge (albeit not pure) could occupy spiritual needs of men whereas kings with all their money and military could not occupy that part of the soul with material lusts of this world.
If you could comprehend the above, then you would little wonder how Islam -with its pure divine knowledge and spirituality- conquered the hearts before land and spread faster than a huge fire in a forest, where no earthly kings or army could stand in front. Thus, Allah made the Muslim army spread Islam in the lands of the Roman empire as well as the Persian at the time of the companion of the prophet Muhammad peace be upon him.
Now, leave you with Quran, Chapter 3: 110
You are the best nation produced [as an example] for mankind. You enjoin what is right and forbid what is wrong and believe in Allah . If only the People of the Scripture had believed, it would have been better for them. Among them are believers, but most of them are defiantly disobedient.