In the second half of the 20th century, the USSR and US superpowers were engaged in the Cold War, which can be seen as a struggle between hegemonies for global dominance. After 1945, the United States enjoyed an advantageous position with respect to the rest of the industrialized world. In the Post–World War II economic expansion, the US was responsible for half of global industrial output, held 80 percent of the world's gold reserves, and was the world's sole nuclear power. The catastrophic destruction of life, infrastructure, and capital during the Second World War had exhausted the imperialism of the Old World, victor and vanquished alike. The largest economy in the world at the time, the United States recognized that it had come out of the war with its domestic infrastructure virtually unscathed and its military forces at unprecedented strength. Military officials recognized the fact that Pax Americana had been reliant on the effective United States air power, just as the instrument of Pax Britannica a century earlier was its sea power. In addition, a "unipolar moment" was seen to have occurred following the collapse of the Soviet Union.