The aftermath of World War II was the beginning of a new era. It was defined by the decline of the old great powers and the rise of two superpowers: the Soviet Union (USSR) and the United States of America (US), creating a bipolar world. Allied during World War II, the US and the USSR became competitors on the world stage and engaged in what became known as the Cold War, so called because it never boiled over into open war between the two powers but was focused on espionage, political subversion and proxy wars. Western Europe and Japan were rebuilt through the American Marshall Plan whereas Eastern Europe fell in the Soviet sphere of influence and rejected the plan. Europe was divided into a US-led Western Bloc and a Soviet-led Eastern Bloc. Internationally, alliances with the two blocs gradually shifted, with some nations trying to stay out of the Cold War through the Non-Aligned Movement. The Cold War also saw a nuclear arms race between the two superpowers; part of the reason that the Cold War never became a "hot" war was that the Soviet Union and the United States had nuclear deterrents against each other, leading to a mutually assured destruction standoff.