French Emperor Napoleon Bonaparte, needing to finance his wars across Europe, sold all remaining French possessions in America to the United States in 1803. This land went from New Orleans, up the Mississippi River, and across to the Rocky Mountains. President Thomas Jefferson had already sent Meriwether Lewis and William Clark at the head of an expedition to explore much of this territory for the US government in late 1802. Lewis and Clark actually went further than the new Louisiana Territory, and made the Pacific.

President James Monroe outlined the Monroe Doctrine in his seventh State of the Union in December 1823.  Responding to various colonial revolutions throughout the Americas, Monroe wished to establish solidarity with the young republics in North and South America, as well as protecting American interests from European incursions.

The immediate cause of the United States’ declaration of war in April of 1917 was the resumption of German submarine warfare in the early part of 1917.  However, the sinking of the Lusitania, the Zimmerman telegram, and general support for Britain all helped.  News from Russia had little impact in American support for war, and the Russian Revolution took place after America had entered the War.

The Stimson Doctrine, named after the Secretary of State, Henry Stimson, was issued during the Herbert Hoover administration. It was issued in response to Japan’s invasion of China and the threat of similar territorial expansion in Europe. The Stimson Doctrine states that any territory gained via the use of military force should not be considered valid. It has remained a part of United States’ government policy ever since, although the United States itself has hardly been exempt from using force to expand its territory. It was referenced often as a counter to actions taken by the Soviet Union during the height of the Soviet Empire. Historically, and contemporarily, it has been frequently criticized for achieving little except the alienation of the Japanese. The Kellogg-Briand Pact officially denounced warfare, so could be seen as a reasonable answer here, however it occurred several years earlier and did not specifically refer to how the government should consider territorial gain. The other doctrines generally express ideas contrary to the Stimson Doctrine. 

From 1953 to 1979, Iran was ruled as a monarchy by the Shahs, members of the Pahlavi family, with immense Western backing. Under economic and societal pressures, a revolution broke out against the Shah in 1979, led by Islamic radicals under the leadership of Ayatollah Ruhollah Khomeini. Deeply disdainful of Western powers, and responding to anger about the United States' global presence and involvement in the Shah's government, the worker at the American embassy in Tehran were held hostage from November 4, 1979, to January 20, 1981. The Americans were released despite the unwillingness to negotiate on the Ayatollah's part, only as Ronald Reagan was sworn in at his inauguration.

Germany had, in 1917, resumed its unrestricted submarine warfare in the North Sea and the Atlantic. This threatened United States trade interests and, more significantly, resulted in the deaths of dozens of U.S. civilians when the Lusitania was attacked. Although affinity for the English and economic rivalry with the Germans played a part in the Congressional debates, it was Germany’s use of U-Boats that finally convinced Wilson that war could not be avoided.

The Lend-Lease policy, officially "An Act to Promote the Defence of the United States," was effectively the end of the United States' neutrality in World War II. While it did not officially enter the war, the Act made the United States firmly support the Allies, Great Britain, Free France, and the Soviet Union, with economic aid and war material. From its enactment in March 1941 to the end of World War II, the US supplied the Allies with over  in aid and supplies.

The events leading up to WWI are a little crazy in that, taken piecemeal they seem relatively small, but put together, they created one of the most deadly conflicts in history. At any rate, Gavrilo Princip, a member of the Black Hand assassinated Archduke Franz Ferdinand (of Austria-Hungary—and, coincidentally, the name of a modestly successful band) and his wife. This led to A-H issuing an ultimatum to Serbia, which led to escalation and war, which led to the various different treaties coming into play, which, in turn, led to countries that were not even remotely involved in the original conflict becoming embroiled in a massive war.

The entirety of the referenced line reads as follows:

"Yesterday, December 7th, 1941 -- a date which will live in infamy -- the United States of America was suddenly and deliberately attacked by naval and air forces of the Empire of Japan."

On December 7th, 1941, the Japanese attacked the US Pacific Fleet, launching America into World War II.

The Cuban Missile Crisis was a showdown between the United States and the Soviet Union over nuclear missiles in Cuban locations during October of 1962.  John F. Kennedy was president from 1961 until his death in November of 1963.