When bigining war, what happen if know everything, military power, spirit power, conditions, military operation etc, with each others? That is only power and number decide victory or defeat. But, real situation have no intelligence. That is, unknowing lead to feel about for. So thing of indispensable are spy, strategy, tactics. Valid measure are to be carelessness, to attack part of weakness, to take person by surprise, etc. And we imagine many different attack and defense by enemy, and we prepare ourself to cope with these things.