In 1844 James K. Polk of Tennessee was elected president of the United States. He was one of the hardest-working presidents ever. And like his hero Andrew Jackson, he believed in his country's right to lands in the West. Polk wanted Oregon and Mexican-owned California, and so did most other Americans . The land was enticing, and there was something that convinced people that it was right to take it. It was an idea called manifest destiny. Those highfalutin words were first used in 1845 by a newspaper reporter named John O'Sullivan who wrote: "[It is] our manifest destiny to overspread the continent allotted by Providence for the free development of our yearly multiplying millions ."

Like most Americans, Polk believed that the United States had the right and duty to spread democracy across the continent. That phrase—manifest destiny—was on everyone's tongue. No one wanted to admit that greed might have something to do with the drive to take more land. Manifest destiny made it all sound noble. The politician Ignatius Donnelly would later put it this way: "Nothing less than a continent can suffice as the basis and foundation for that nation in whose destiny is involved in the destiny of mankind."

In 1846 Polk signed a treaty with Great Britain giving England the territory which today is western Canada. The U.S. took what is now Oregon, Washington, Idaho, and parts of Montana. Then President Polk went to war against Mexico . The war lasted for two years . When it ended, the border between Texas and Mexico was set at the Rio Grande. But that wasn't all the United States got. It got California—which at that time meant land that stretched from Texas to the Pacific Ocean and as far north as Wyoming .

President Polk didn't know just how lucky he was. Nine days before the war's end, something was discovered in California on land that belonged to a good-hearted Swiss man, John A. Sutter. Something very valuable. Gold !