养兵千日，用兵一时

长期供养、训练军队，是为了关键时刻用来打仗。其深层含义包括两方面：一方面，就国家而言，军队建设是一项需要长期持续努力的工作，不能因为耗资巨大、很少打仗等而松懈；另一方面，就军队或官兵而言，关键时刻要挺身而出，忠诚勇敢地担负起自己的职责。引申含义则超出军事领域，泛指长期准备或平时积蓄力量，在关键时刻或必要时发挥作用。

Maintain an Army a Thousand Days for Use at a Critical Time

The purpose of raising and training an army for a long time is to use it for warfare at a critical time. The significance of this concept has two aspects: one is that for the country, building of the military is a long-term and continuous effort, which should not slacken because it is costly and armed battles are rare; the other is with regard to the officers and soldiers, who should be ready to come forward at critical moments and take up their duties faithfully and bravely. The extended meaning goes beyond the military and refers to long-term preparation or the accumulation of strength in normal times for use at critical moments or whenever necessary.

引例

朝廷养军千日，用在一时。汝安敢出怨言，以慢军心！（罗贯中《三国演义》第一百回）（朝廷养兵千日，用兵一时。你怎么能口出怨言，涣散军心呢！）

The imperial court maintains an army for a thousand days to be used at a critical moment. How dare you speak like this to demoralize the army? (Luo Guanzhong: Romance of the Three Kingdoms)