5 - President Lincoln releases John Slidell and James Mason Ending the Trent Affair (1861).
This early diplomatic row between the United Kingdom and the United States inflamed popular opinion in the UK against the United States at a time when the Civil War was still in its infancy and the cause of the Union far from success. The decision by an American ship's captain to seize the Confederate diplomats Slidell and Mason off of the Trent, a British ship carrying them to the UK, proved too provocative for the British to slough off. After weeks of talk of war and rising tensions, the President decided to release the Confederates. They ultimately failed in their goal to achieve diplomatic recognition for the Confederacy. By releasing them, President Lincoln averted what might have been a fatal (to the cause of the Union) UK intervention into the Civil War on the side of the Confederacy.
4 - Alaska Purchase (1867)
Secretary of State William Seward's belief that purchasing the Alaska territory from the Russian Empire was a great idea - and it certainly was. The natural resources of Alaska notwithstanding, I would argue that a Russian Alaska would have aggravated Red Baiting in the 1950s, destabilizing American culture and taking this country in a dangerous direction. Soviet nuclear weapons deployed in the Alaskan Panhandle? No thanks.
3 - Signing the Antarctic Treaty
By signing the Antarctic Treaty in 1959, the US and the other signatories placed Antarctica under global stewardship. While this has not always been a perfect solution or easy, it speaks to the capacity of nations to put global concerns ahead of national ones. In the 21st century, global stewardship of resources might be our best hedge against resource warfare.
2 - Reconstructing Japan and Germany After World War II (1945)
The United States, taking the lead in the post-war period in a way it failed to after World War I, put in place systems in both Japan and Germany that lead to two critical features of the late 20th century. First, both of these countries were effectively demilitarized in practice and, most critically, in culture. The postwar economic boom in both countries in the 1950s stimulated the rise of the Japanese and German middle classes and a cementing of those countries into Western systems of governance. This was a tremendous boon to the United States in the generation after the war - when industrial-age America was at the zenith of it's power to provide a better life for its people.
1 - Louisiana Purchase (1803)
It is hard to imagine the United States becoming the country it became if President Jefferson did not take Napoleon's offer to sell the entire Louisiana Territory in 1803, despite Jefferson's constitutional principles. Louisiana guaranteed the long-term security of the country while also demonstrating the idea that the Constitution should not be read narrowly but rather interpreted to serve the needs of the age.