My usual answer is that, for maximum nuclear proliferation, have the WAllies win WWII and the USSR not dominate Europe. Without the Cold War, the US will fall into a spirit of isolationism similar to what happened after WWI and decides that she's only going to play in her backyard(Latin America and the Pacific) from now on. There's no NATO or Warsaw Pact(or global system of alliances in general) and ATL's equivalent of the Marshall Plan(In the TL I'm working on, it's the Vandenberg Plan) is comparatively paltry, speeding up decolonization. Pretty soon, the nations of the world realize that only they can protect themselves, and one option is singularly attractive: the nuclear bomb.