Not just Americans and Australians, but many people around the world feared Communist infiltration because they saw what they viewed as the relentless expansion of communism around the world. In the immediate aftermath of World War II, the Soviet Union had more or less installed communist governments throughout Eastern Europe, an "iron curtain" that stretched from Poland to Yugoslavia (a nation that had established a communist state independently of the USSR.) The Soviet Union successfully detonated an atomic bomb in 1949, and Americans became aware that they had gained much of the intelligence they needed to develop this weapon by the use of spies within the Manhattan Project. This brought fears of communism home in ways that nothing else had. Moreover, communism seemed to be spreading. The most populous nation in the world, China, "fell," from a western perspective, to communism, as Mao Zedong and the communists emerged victorious from a bloody civil war. This especially concerned Australians, and their fears that communism would spread to their front door, as it were, seemed realized when communist North Korea invaded the western-backed South in 1950. All of this, of course, was based on the assumption that communism and Western-style governments, rooted in political democracy and capitalism, were incompatible on a global scale. So bearing this in mind, people in places like the United States and Australia saw communism as a real and immediate threat in ways that are difficult to understand today.