It is almost an undeniable assumption that the alliance of the USSR and USA was, as historian Caroline Kennedy-Pipe says, it was an ‘alliance of desperation, not trust’, and thus it would appear one of convenience, rather than voluntarial. One of the more simplistic reasons for the alliance was the need to defeat a common enemy – by uniting, it would be both easier and more efficient to work together to achieve a common goal. Stalin was afraid of German invasion, and so the Nazi-Soviet Pact signed on the 28th August, 1939 was a promise of neutrality between each other if either country became involved in a war. However, the German invasion of Russia in 1941 made it clear to Stalin that Hitler was never true to his promises, and so turned to the allies – who were the only option of help he had left. This supports the point that Stalin never wished to assist the allies in the first place – the open hostility and distrust (on both sides) didn’t help with an alliance beforehand. Stalin opted in for an alliance with the allies so as to take pressure off his western front, so that the USSR wouldn’t be overwhelmed by two front lines – the other being from the Japanese in the east. To further the problem, the US was only allies with the Soviet Union because (after joining the war later than when Stalin decided to join the allies) the existing allies already were.
An alternative explanation is the ideologies involved