The Cold War was the continuing state of conflict, tension and competition that existed primarily between the United States and the Soviet Union and those countries' respective allies from the mid-1940s to the early 1990s. Throughout this period, the conflict was expressed through military coalitions, espionage, weapons development, invasions, propaganda, and competitive technological development, which included the space race. The conflict included costly defense spending, a massive conventional and nuclear arms race, and numerous proxy wars; the two superpowers never fought one another directly.