From the most fundamental of positions, neither the United States or the Soviet Union really trusted one another. Their alliance in World War II was out of convenience. Both saw Hitler as a threat and both needed the other the negate it. The Soviet Union signed a nonaggression pact with the Nazis and only when Hitler invaded Russia did the Soviet Union ally itself with the United States. The mutual need to eliminate Hitler was an excellent source of cohesion. When the threat was ultimately eliminated, old tensions resurfaced. There was considerable mistrust about what a post- Hitler Europe would look like. Stalin did not want to have another attempted invasion of Russia, so his desire to create a series of buffers between it and the rest of Europe was seen as expansionist by the West. The threat and use of nuclear weapons enhanced the fundamental mistrust between both nations. These tensions which were exacerbated by the peace conferences following the Nazi defeat ended up forming the basis of the Cold War that followed World War II.