MORE LAND FOR AMERICA
Destiny hadn't finished manifesting itself at the beginning of the 1840s, but shortly thereafter, the United States picked up a lot of land in a short time.
First came the Aroostook War (1842). These lumberjack battles in northern Maine were settled with the British masters of Canada in a way that left Maine with plenty of north woods, Canada with room to build a winter road to Quebec, and the U.S. with the nice surprise of major iron deposits in Minnesota.
Then Texas won its independence from Mexico in 1836, but between then and 1845, Texans had to stand by while Congress debated whether to admit it as a slave state. In 1845, Texas switched from being the Lone Star Republic to being the 28th star in the American flag. Mexico couldn't stand the transition of its former territory to the United States, and war broke out soon after American provocations in disputed south Texas.
One of Andrew Jackson's last political acts was recommending his friend and neighbor James Polk as the Democratic presidential nominee in 1844. Polk was victorious over a terminally frustrated Henry Clay and efficiently carried out his four-point presidential agenda: lower tariffs, establish an independent treasury, grab California (looked beautiful even before gold was discovered in 1848), and settle the Oregon border. Polk got legislation to lower the tariffs and restore an independent treasury. Then he went to work on grabbing California and the Oregon Territory .
In 1846, the British settled the Oregon Territory question by splitting the difference with the United States on the northern border of what became the state of Washington.
Then Polk managed to stir up border incidents with an angry Mexico down on the Rio Grande in Texas, and the Mexican-American War was under way in 1846. After some tough fighting, the U.S. defeated Mexico and in 1849 got California and all of the Southwest. At the time, California's inhabitants included around 13,000 Mexicans, 1,000 Americans more or less poaching on Mexican territory, and 100,000 American Indians. The war was unpopular in New England; Henry David Thoreau spent a night in jail to protest it. Abe Lincoln opposed it.
Most of the rest of the country was spoiling to finish off Manifest Destiny with a good fight. Renegade Americans in California seized the province, aided by Captain John Fremont, who just happened to be in the neighborhood with an armed patrol.
An American army fought its way into Mexico City by September 1847. Mexico reluctantly signed the Treaty of Guadalupe Hidalgo (1848), turning over an area that amounted to about a third of the United States in exchange for $18 million (offered rather guiltily by the United States). When the treaty was signed, neither side knew that gold, ultimately worth billions of dollars, had been discovered nine days earlier on the American River in California.