Principle number 7 of Basic Elements of Islam: The idea-collection provides new soldiers by allowing polygamy. According to Sharia law, a Muslim man can marry up to four wives, and he can have sex with as many slave girls as he wishes.
The Quran especially encourages men to marry widows. This is an important idea to add if you are going to be losing a lot of soldiers in war. You need some way of replenishing your army. Otherwise the idea-collection could die out from a lack of offspring. Read more about this principle.