The Aftermath of World War II was the beginning of a new era, defined by the decline of all European colonial empires and simultaneous rise of two superpowers: the Soviet Union (USSR) and the United States (USA). Allies during World War II, the US and the USSR became competitors on the world stage and engaged in the Cold War, so called because it never resulted in overt, declared hot war between the two powers but was instead characterized by espionage, political subversion and proxy wars. Western Europe and Japan were rebuilt through the American Marshall Plan whereas Central and Eastern Europe fell under the Soviet sphere of influence and eventually behind an "Iron Curtain". 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 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.