How did the Gold Rush affect westward expansion?

In 1846, while tensions between the U.S. and Mexico over Texas exploded into war, American forces under Gen. John C. Fremont also laid claim to California. White Americans who had settled in California referred to their effort to end Mexican rule as the Bear Flag Revolt. (The California state flag still depicts a grizzly bear.) American troops aided this revolt, and in August, the U.S. declared that it had annexed California. Some Mexican residents rebelled against the U.S. forces and soon overthrew the military government that the U.S. had hastily established. By the end of 1847, however, the U.S. prevailed, gaining control of the vast territory of California.

On January 24, 1848, James Marshall discovered gold at Sutter's Mill along the American River near Sacramento, California. When word of Marshall's discovery reached the East Coast in August, it touched off the largest Gold Rush in history. More than 200,000 settlers flooded into California over the next four years. More than ninety percent of these ?49ers were men, who hoped to make their fortune prospecting for gold. Nearly 50,000 Chinese also immigrated across the Pacific Ocean to California between 1848 and 1854, where they prospected, mined, laid railroad tracks, and performed other jobs.

The sudden growth of California's population made statehood an urgent issue. As with Texas a few years earlier, the debate over California forced the issue of slavery into the very center of American politics.