Why did the Cold War start?
Although different people might answer this in different ways, I would argue that the Cold War started because the United States and the Soviet Union did not trust one another. Because they did not trust one another, each interpreted the other’s actions in the most negative possible light and each thought the other was a grave threat.
The Soviet Union and the US never really trusted one another. The Soviet Union’s communist ideology specifically stated that the capitalists were the enemy of the workers and needed to be overthrown. Therefore, they were inclined to distrust the capitalist Americans. Moreover, the US had acted in ways that made the Soviets trust them even less. The US had sent at least some troops to help fight against the communists in the Russian Civil War after WWI. The US had refused to recognize the USSR for decades. The US had, in the Soviets’ minds, dragged its feet on invading Europe during WWII, allowing the Red Army to lose millions of men fighting the Germans. All of these things predisposed the USSR to distrust the Americans.
The Americans, in turn, distrusted the Soviets. Communist ideology explicitly called for world revolution, which meant that the US felt that the USSR was dedicated to overthrowing the US political and economic system. The US also believed that the communists were going to try to spread their influence to other countries of the world. Because of this, they were very suspicious when the Soviets wanted to control Eastern Europe as their own “sphere of influence” or buffer zone of satellite states. The Soviets felt this was sound strategy, but the US believed that it meant that the Soviets were trying to dominate Eastern Europe as a first step towards taking control of Western Europe.
Because each side in this situation distrusted the other side, conflict was bound to arise. The Cold War began because the two sides did not trust one another and because each felt the other side was trying to destroy it.