Задний ум хорош, да никуда не гож

It is of no help saying what should have been done to prevent some trouble after it took place. See Задним умом дела не поправишь (3), После драки кулаками не машут

(77)

Cf: After-wit is dear bought (Br.). Hindsight is better than foresight (Am.). If a man's foresight were as good as his hindsight, we would all get somewhere (Am.)