Arriver comme les carabiniers

Literally: To arrive like the carabineers

Example: “Les renforts sont arrivés comme les carabiniers, une fois l'incendie éteint.” — The reinforcements showed up a day late and a dollar short, once the fire was already out.

English equivalent: A day late and a dollar short

Origin: From Offenbach's 1869 opéra-bouffe (comic opera) Les Brigands, whose carabineers cheerfully sing that they are the safety of the nation but, by some unfortunate chance, always arrive too late. The joke outlived the operetta and became the standard French way of mocking a late rescue.