Why did the cold war start? at the end of world war ii, the soviet union and the united states competed for global influence and power. at the start of world war ii, hitler's pact with stalin forced the united states to attack the soviets before they joined the allies. at the end of world war ii, stalin believed that the united states and other western democracies were preparing to attack. at the end of the korean war, the soviet union believed the united states had successfully stopped the spread of communism in asia.