From Liddel Hart's "Why We Don't Learn From History" -

Faith matters so much to a soldier, in the stress of war, that military training inculcates a habit of unquestioning acceptance of the prevailing doctrine. While fighting is a most practical test of theory, it is a small part of soldiering; and there is far more in soldiering that tends to make men the slaves of a theory.

Moreover, the soldier must have faith in his power to defeat the enemy; hence to question, even on material grounds, the possibility of successful attack is a risk to faith. Doubt is unnerving save to philosophic minds, and armies are not composed of philosophers, either at the top or at the bottom. In no activity is optimism so necessary to success, for it deals so largely with the unknown—even unto death. The margin that separates optimism from blind folly is narrow. Thus there is no cause for surprise that soldiers have so often overstepped it and become the victims of their faith.

I think an unheralded reason for the success of Christianity and Islam is the belief that dying for the religion is the greatest possible victory. Thus, the religious warrior had a win/win position - the enemy would be destroyed, raising up his civilization; or his body would be destroyed, and he would enter paradise as a blessed martyr.

Other cultures have been able to achieve this, like the Japanese samurai culture, which too had a spirit of win/win towards combat, and dying in it.

Yukio Mishima wrote that "[the Japanese] image of death [is that there exists beyond life] a spring of pure water, from which tiny streams continuously pouring their pure waters into this world." To die was to re-join the stream of pure water, leaving behind honor and a glorious ending upon your bon-voyage to life.

For those of us without a philosophical or theological tradition that glorifies action for the cause, a careful balance must be struck between rational skepticism with the maximization of willpower and strength. Doubt is unnerving save to philosophic minds -- yet skepticism is what allows us to break free from the past.