Principle number 15 of Basic Elements of Islam: A woman is in a thoroughly subordinate position. This idea really helps support other ideas in the collection. If women had too much influence, they'd try to curb the warring. Women in general don't like to send their husbands and sons off to war. But if women have no say in the matter, then the rest of the ideas can express themselves without interference. By subordinating women, the idea-collection prevents their effective vote against war, violence, and conquest.
The rules and laws within Islam that keep women subordinate are numerous. For example, she is not allowed to leave her house unless she is accompanied by a male relative. Under Islamic law, a woman is forbidden to be a head of state or a judge. She can only inherit half of what a man can inherit. In court, her testimony is only worth half of a man's. She is not allowed to choose where she will live or who she will marry. She is not allowed to marry a non-Muslim or divorce her husband. He, however, can divorce her with a wave of his hand. And according to Sharia, he can (and should) beat her if she disobeys him.
All of these ideas keep her subordinate, which helps keep the war machine going unimpeded by domestic rebellion. Read more about Islam's subordination of women.