The Cold War was a period of geopolitical tension between the United States and the Soviet Union and their respective allies. The first phase of the Cold War began after the end of World War II in 1945. By the 1980s, another period of heightened tension arose, leading the United States to increase diplomatic, military, and economic pressures against the Soviet Union. During this time, an error in the Soviet Union Oko system almost triggered a nuclear war between the two military superpowers.