Which event or circumstance do you consider to be the most significant? why?

The most major event i believe is "The bombing of Pearl Harbor". Without this, the US would most likely never have joined the war militarily with sufficient allies left, and the US would face a two front war by itself when Britain fell (which would have been sooner or later).With the US joining the war, the Allies changed their plans from stalling the enemies to actually fighting back

hope this helps