Your challenge is to make Britain and France declare war on Germany over the occupation of Czechoslovakia in March 1939 (this presumably implies the necessity of an Anglo-French guarantee of rump Czechoslovakia against Germany following Munich, but if you can find another way around this feel free to do so). How does this happen? And what happens next: rapid Anglo-French military reaction? A phony war, followed by a German strike west after a few months of integrating the Czechoslovak arsenal? Something completely different? How do you think the conflict might proceed in each of those possible scenarios? There are no real constraints on the point of divergence, but it should preferably be as late as possible.