<P> The feasibility of an attack on the continental United States by Imperial Japan was considered negligible, with Japan possessing neither the manpower nor logistical ability to successfully mount a full - scale invasion of the U.S. Minoru Genda of the Imperial Japanese Navy advocated invading Hawaii after attacking Pearl Harbor, believing that his country could use Hawaii as a base to threaten the mainland United States, and perhaps as a negotiating tool for ending the war . The American public in the first months after the attack on Pearl Harbor feared a Japanese landing in California and reacted with alarm to a rumored raid in the Battle of Los Angeles . </P> <P> During the war, Japan successfully occupied, but later withdrew from, the Aleutian Islands of Alaska . Japan also conducted air attacks through the use of fire balloons . Six American civilians were killed in such attacks; Japan also launched two manned air attacks on Oregon (the only time in the war that mainland America was bombed by enemy aircraft) as well as two incidents of Japanese submarines shelling the American mainland . </P> <P> During the Cold War, the primary threat of an attack on the United States was viewed to be from the Soviet Union . In such an attack, nuclear warfare was projected to almost certainly happen, mainly in the form of intercontinental ballistic missile attacks as well as Soviet Navy launches of SLBMs at U.S. coastal cities . </P> <P> The first Cold War strategy against a Soviet attack on the United States was developed in 1948, made into even firmer policy after the Soviet development of the atom bomb in 1949 . By 1950, the United States had developed a defense plan to repel a Soviet nuclear bomber force through the use of interceptors and anti-aircraft missiles, while at the same time launching its own bomber fleet into Soviet airspace from bases in Alaska and Europe . By the end of the 1950s, both Soviet and U.S. strategy included nuclear submarines and long range nuclear missiles, both of which could strike in as little as ten to thirty minutes while bomber forces took as long as four to six hours to reach their targets . The concept thus developed of the nuclear triad where all three weapons platforms (land based, submarine, and bomber) would be coordinated in unison for a devastating first strike, followed by a counterstrike, accompanied by "mopping up" missions of nuclear bombers . </P>

When was the last time that the united states was invaded