Union, much of the war-fighting doctrine held by western nations revolved around using a large number of smaller nuclear weapons in a tactical role. It is debatable whether such use could be considered "limited" however because it was believed that the United States would use its own strategic weapons (mainly bombers at the time) should the Soviet Union deploy any kind of nuclear weapon against civilian targets. Douglas MacArthur, an American general, was fired by President Harry Truman, partially because he persistently requested permission to use his own discretion in deciding whether to utilize atomic weapons on the People's Republic of China in 1951 during the Korean War. Mao Zedong, China's communist leader, gave the impression that he would welcome a nuclear war with the capitalists because it would annihilate what he viewed as their "imperialist" system. The concept of a "Fortress North America" emerged during the Second World War and persisted into the Cold War to refer to the option of defending Canada and the United States against their enemies if the rest of the world were lost to them. This option was rejected with the formation of NATO and the decision to permanently station troops in Europe. In the summer of 1951, Project Vista started, in which project analysts such as Robert F. Christy looked at how to defend Western Europe from a Soviet invasion. The emerging development of tactical nuclear weapons was looked upon as a means to give Western forces a qualitative advantage over the Soviet numerical supremacy in conventional weapons. Several scares about the increasing ability of the Soviet Union's strategic bomber forces surfaced during the 1950s. The defensive response by the United States was to deploy a fairly strong "layered defense" consisting of interceptor aircraft and anti-aircraft missiles, like the Nike, and guns, like the M51 Skysweeper, near larger cities. However, this was a small response compared to the construction of a huge fleet of nuclear bombers. The principal nuclear strategy was to massively penetrate the Soviet Union. Because such a large area could not be defended against this overwhelming attack in any credible way, the Soviet Union would lose any exchange. This logic became ingrained in American nuclear doctrine and persisted for much of the duration of the Cold War. As long as the strategic American nuclear forces could overwhelm their Soviet counterparts, a Soviet pre-emptive strike could be averted. Moreover, the Soviet