The Cold War was a period of hostility between the United States and Soviet Union from the late 1940s to early 1990s. It arose due to tensions between capitalist West and communist East as communism spread across Eastern Europe and Asia following WWII. The Cold War was fought through arms races, space races, sports competitions, propaganda, and proxy wars as each side aimed to contain the influence and military power of the other through maintaining nuclear deterrence and competing for dominance globally and in new technologies. Ultimately, the Soviet Union's communist system proved economically unsustainable and collapsed in 1991, with the US emerging as the sole superpower.