I've been a reader here for a while, but I've only just started posting, so sorry if this has been covered before...
In the early days of WWII, especially after the Molotov-Ribbentrop pact and the Soviet invasion of Finland, the British viewed Stalin and the U.S.S.R. as an ally of Hitler and the Third Reich. As a result, a plan to attack the Soviet Union and disrupt German petroleum supplies by destroying the oilfields at Baku -
Operation Pike - was drawn up and almost (depending on who you ask) carried out by the Anglo-French alliance in early 1940.
Suppose that the plan is carried out, and the R.A.F. bombs the Baku oil-fields, leading to a state of war between the Soviet Union and Western Allies. What would happen next?
Things are going to get pretty messy relatively quickly afterwards. I realize this won't turn into a Molotov-Ribbenwank

D) where the Germans and Soviets carve up the whole world together, but it's still going to be interesting. After all, they'll be fighting on the same side, but an alliance won't last - Hitler needs his "lebensraum" in the long run...