- Polk was the penultimate slaveholding president (Zachary Taylor was the last). He used his increased income from being President to buy more slaves.
- Polk aggressively pursued Manifest Destiny.
- He started a war with Mexico with the intention of taking California, and the territory to the east, from Mexico.
- He tried to buy Cuba from Spain but failed.
- He threatened war with Britain (Fifty-four Forty or Fight) and demanded half of what is now Canada but settled for Oregon, Washington, and Idaho.
The Mormon Battalion
Polk had defended Mormons against religious persecution and even donated money ($10, I didn't say Polk wasn't a cheapskate) to the church. In 1846, Polk recruited the only religiously based unit in United States military history. As a military outfit, the Mormon Battalion marched a lot (from Iowa to San Diego) but was otherwise useless. They did fight one battle against a herd of wild cattle (the victory is commemorated with a monument).
Polk made a secret deal with Brigham Young to turn over to Mormons the newly acquired land around the Great Salt Lake in exchange for their allegiance to the United States government. The salaries paid to the Mormon Battalion amounting to a bribe for that loyalty.