As expected, the Allies did declare war when Hitler invaded Poland as they had refrained from doing in reaction to his other land grabs.
That said, it is an interesting question whether the war could have been headed off had the Allies stopped appeasing Hitler at an earlier date.
Clemenza was right...
“You know, you gotta stop them at the beginning. Like they should have stopped Hitler at Munich, they should never let him get away with that, they was just asking for trouble.”