Your alternate history challange if you accept it is to have a one world goverment by 1970, with a P.O.D. after 3000 B.C. (It's in after 1900 because thats wen th end date is. if it sould be in before 1900 move it there IAN the Admin)
Howabout this:
OTL 3000BCE---1942 CE
Hitler goes through with Code Blue, the proposed invasion plan for the USSR that goes south, through the Ukraine and Stalingrad. The rapid success of this operation means the by late 1942, Stalingrad has fallen and the USSR is cut off from oil and food. However, Stalin fights on, with still numerous resources from Siberia, and aid from Britian and the USA. Fronts are still opened in Italy and Greece by the allies, but they lack the forces to land at Normandy,
The Pacific front goes generally OTL, ending with a US use of the atomic bomb. The USA then threatens to use the atomic bomb on Germany. Hitler refuses to surrender. By this time, bombings have already hurt Germany, with the majority of the Luftwaffe stretched to the breaking point in the stalemate the is the Eastern Front, with the USSR holding on due to allied aid.
In early 1946, the United States destroys Berlin with an atomic bomb. Hitler is killed. Many German generals, seeing the writing on the wall, surrender to the allies.
The Post-War world is essentially Pax-Americana. The USA holds Europe in monetary control, and pushes free trade and de-colonization. After much agreements, the European powers decolonized slowly, as per an agreement with the USA. The USSR is a rump of it's former self, and the axis is defeated. The USSR is defeated to such an extent, that there is no Cold War. The UN is allowed to function moderatly well, and with free trade, the world slowly unites under UN jurisdiction. While the UN is not a main federal body, it is much more powerful than OTL.
I'm sure I overlooked something obvious, though.