Deuteronomy 23:9-11. Among the rules that God gives to Moses is this one: when the Israelite men go forth against their enemies, they must keep themselves "from every evil thing." If any of them become unclean by what "chances to happen by night," then he must go outside the camp for the entire day. When evening comes, he shall bathe himself in water and when the sun goes down he may return to camp.

God knows us so well! He knows that a fighting force is weakened when thoughts (or even dreams!) of sex creep in. He knows men will be tempted to do things they would not normally do when they are away from home and encamped with other men for a long period of time. He knows that not only boredom and loneliness, but also the thrill of battle and victory (or the agony of defeat) will bring thoughts or urges for sex, as well. So He cuts that off from the very beginning. No Israeli soldier shall be in any way "unclean" while in camp, even if it is just from having an unclean dream.

If the soldiers know this rule and that it has no exceptions, they can put those thoughts out of their mind as soon as they pop in, and they will have the support of their fellow soldiers, all of whom are following the same rule. Thus, the Israeli army will be formidable since other armies did not have anything like this - in fact sex was often a spoil of war, and homosexual relations among men were tolerated. The Jewish army will be focused and ready to fight while their enemies will often have the most distracting of other things on their minds!