After World War II, the world was left to question what was next for geopolitical health. Because of this newfound power vacuum, two powers - the United States and the Soviet Union (USSR) - grew to become great enemies. These two nations fought mainly on the basis of their economic systems, with the Cold War being fought between NATO, a capitalist group of western countries led by the USA, and the Warsaw Pact, a communist group of eastern European countries led by the USSR. Although the USA and USSR never fought directly, proxy wars such as those in Vietnam and Korea heightened tensions along with the building of the Berlin Wall to keep defectors from East Germany from defecting. The Cold War finally came to the end nearly 50 years after it began with the fall of the USSR in 1991 to form the Russian Federation.