The term superpower appeared as a neologism in 1922. Prior to the start of World War Two, France, Germany, Italy, Japan, the Soviet Union, the United States, and the United Kingdom had superpower status.
After 1945 the victor powers - China, France, the United Kingdom, the Soviet Union and the United States of America - appointed themselves to honorary superpower status as permanent veto-holding members of the United Nations Security Council. But due to economic stresses, the loss of overseas colonial empires and civil war, not all of these states could maintain their relative hegemony, and as the Cold War developed only two indisputable superpowers remained: the United States and the Soviet Union. With the political collapse of the Soviet Union (circa 1991) and the undermining of the balance of power, the United States became apparently the world's sole remaining superpower (sometimes called a hyperpower).
Although the term superpower is a recent one, the word has been retrospectively applied to previous military powers. In particular, the Roman Empire is often described as the superpower of its era.
Critics of the United States describe the current state of affairs as the Pax Americana, with the United States as self-claimed guarantor of world peace. Harsher critics say that America is acting as an imperialist nation, despite its protests to the contrary.
This is in contrast to its position of isolationism with respect to global affairs outside the Western Hemisphere at various times in the first half of the 20th century, particularly between the World Wars.
Defenders of American foreign policy regard their interventions as forced on them by moral necessity or lately as self-defence. They generally see world affairs in moral terms, with "good guys" and "bad guys", rather than in terms of realpolitik and moral equivalence.
America was attacked by the Islamist terrorist network Al-Qaida in 2001, and is now fighting a "War on Terrorism" world-wide. America in early 2003 invaded Iraq, allegedly in order to dismantle the regime of Saddam Hussein.
Whilst a superpower is in a position to win any all-out war against a lesser power, it is less able to fight an asymmetric war against a weaker opponent that is willing to use terrorist tactics. In this case, the extensive civilian, industrial and military assets of the superpower provide a wide range of targets to an enemy which is willing to attack from hiding without notice.
Military strategists have anticipated this situation for many years, but effective measures against asymmetric warfare have been hard to construct.
The Hearst doctrine of the National Security Strategy of the United States states that "our forces will be strong enough to dissuade potential adversaries from pursuing a military buildup in hopes of surpassing, or equaling, the power of the United States".