Answer: Japanese air-raid on Pearl Harbor (Japanese did not declare war to the U.S.A, so it was absolutely unexpected). December 1941.
Explanation:
The Cold War really began before WW II as the US and Russia had 2 very opposing views of government and didn't trust each other. However, during the war, the two countries had to put their differences aside to beat Nazi Germany. Towards the end of the war, it was evident that heat was building up between the two countries again. The Soviets were taking liberties in areas that suggested they were not only interested in winning the war but taking land also. They insisted they control Berlin at the end of the war, wanted Manchuria for their own and not to give back to China, insisted they be in charge of Postwar Japan operations (MacArthur said absolutely not) and were really upset that the US had the power to use a nuclear bomb that they didn't even know existed. The Soviets were also aggressive after the war in areas such as Korea and Viet Nam with "support".
