In the early nineteenth century, tensions heightened as Spain, France, and Britain vied for influence in South America. On December 12, 1823, President James Monroe spoke to Congress, declaring what came to be known as the Monroe Doctrine, letting Europe know that the Western Hemisphere was no longer open to European Colonization. Monroe declared that any European effort to extend their political influence into the Americas would be considered "as dangerous to our peace and safety." Thus, the United States asserted a dominant interest in the hemisphere.
After World War II, the U.S. and the Soviet Union sparred for influence in the Cold War, often using other nations as proxies. President Harry Truman addressed Congress on March 12, 1947, to counter Soviet influence in Greece and elsewhere. He announced that the U.S. would support "free peoples who are resisting attempted subjugation by armed minorities or by outside pressures," and that Americans must be "willing to help free peoples to maintain their free institutions and their national integrity against aggressive movements that seek to impose upon them totalitarian regimes." The Truman Doctrine began an effort of "containment" of the Soviet Union.
In 1969, after 20 years of the Cold War, and during the height of the Vietnam War, President Richard Nixon announced that the U.S. would continue to be responsible for the deterrence of nuclear and conventional war, but that the responsibility for deterrence of local wars would thenceforth rest with the countries threatened by such wars. The U.S. would continue to assist such countries, but they would be expected to increase spending and use their own troops for their defense.
After the September 11, 2001 attacks on the U.S., President George W. Bush announced that the U.S. would "make no distinction between the terrorists who committed these acts and those who harbor them," noting that "every nation, in every region, now has a decision to make. Either you are with us or you are with the terrorists." Bush later enunciated a policy of pre-emptive war–the right of the U.S. to pursue unilateral military action when acceptable multilateral solutions cannot be found and to strike (make war) when a threat was perceived, to stop a possible attack from occurring.