The Cold War started after the tension the Soviet Union started it wanting to become a world power. They started making more weapons and advances than the U.S. This caused us to want to also expand so that we could stop communism from spreading and protect capitalism. The Soviet Union developed nuclear weapons greater than ours and positioned them off of Cuba at the U.S. These lead to us making nuclear weapons. Eventually we called for an agreement to reduce the amount of weapons we had so that we could peacefully coexist with the Soviet Union.