What was the catalyst that started World War 2?

Hitler's invason of Poland.

Britain and France had threatened to declare war if he crossed the Polish border, but Ribbentrop had persuaded him that neither of these two nations was prepared to fight.

When they duly declared war, Hitler was badly wrong-footed. It was noted that he was now obliged to go to war against Britain, with Russia on his side, when he had expected to do the opposite.