After WWII, tensions grew between the US and USSR as they emerged as rival superpowers. The USSR set up communist governments in Eastern Europe and cut off access to West Berlin, leading to the Berlin Airlift. NATO and the Warsaw Pact were formed as military alliances. During this time, fears of communism spread in the US through events like the Hollywood blacklist and trials of accused spies like the Rosenbergs. The development of nuclear weapons by both sides increased tensions further in an arms race and space race throughout the 1950s.