The Cold War was a period of geopolitical tension between the Soviet Union and the United States and their respective allies, the Eastern Bloc and the Western Bloc, after World War II. The period is generally considered to span the 1947 Truman Doctrine to the 1991 dissolution of the Soviet Union. The term "cold" is used because there was no large-scale fighting directly between the two superpowers, but they each supported major regional conflicts known as proxy wars. The conflict was based around the ideological and geopolitical struggle for global influence by the two powers, following their temporary alliance and victory against Nazi Germany in 1945. Whilst the Western Bloc and Eastern Bloc generally espoused the economic theories of capitalism and socialism, respectively, the conflict was mostly geopolitical in nature. Each power had a nuclear strategy that discouraged a pre-emptive attack by the other side, on the basis that such an attack would lead to the total destruction of the attacker—the doctrine of mutually assured destruction.