The discovery of gold in California capped the most dramatic period of expansion in American history. Under James K. Polk's "Manifest Destiny" presidency, the United States nearly doubled its size by annexing the Oregon Territory south of the 49th Parallel and capturing the northern half of Mexico in the Mexican War. The Gold Rush, by inducing rapid settlement of the farthest reaches of those new American lands, consolidated Polk's territorial gains and made the United States into a true transcontinental empire.
California's application for statehood in 1850, however, revealed just how fragile the political structure holding that empire together really was. The promise of future westward expansion had allowed the partisans of the slave South and free North to defer settlement of the sectional question ever since the ratification of the Constitution. Each side imagined the future settlement of the West with their own kind would eventually settle the slavery question in their favor. California's application to join the USA as a free state, despite its southerly latitude, threatened to upset the delicate sectional balance in the Senate, and proslavery politicians demanded certain concessions from their Northern colleagues in order to accede to California's statehood. The result, the Compromise of 1850, created a free California but imposed a Fugitive Slave Law that inflamed antislavery opinion in the North, beginning the decade-long descent from sectional crisis to the Civil War.