戦を見て矢を矧ぐ

Literally, “to string an arrow upon seeing the fighting”。

戦(いくさ)を見(み)て矢(や)を矧(は)ぐ • (ikusa o mite ya o hagu) 

1. to close the stable door after the horse has bolted (to make preparations only when it is already too late; to act after the critical moment has passed)

• 泥棒(どろぼう)を捕(と)らえて縄(なわ)を綯(な)う (dorobō o toraete nawa o nau)
• 難(なん)に臨(のぞ)んで遽(にわ)かに兵(へい)を鋳(い)る (nan ni nozonde niwa ka ni hei o iru)