Answer:
The California Gold Rush
Sandwiched between the Louisiana Purchase in 1803 and the Civil War in 1861, the California Gold Rush is considered by many historians to be the most significant event of the first half of the nineteenth century.
The discovery of gold at Sutter's Mill on January 24, 1848 unleashed the largest migration in United States history and drew people from a dozen countries to form a multi-ethnic society on America's fringe. The promise of wealth forever altered the life expectations of the hundreds of thousands of people who flooded California in 1849 and the decade that followed. The gold also fired up the U.S. economy and fueled wild dreams like the construction of a cross-country railroad line.
When the United States and Mexico went to war in 1846, California was under the loose control of the Mexican government. California's population consisted of about 6,500 Californios (people of Spanish or Mexican decent), 700 foreigners (primarily Americans), and 150,000 Native Americans, whose numbers had been cut in half since the arrival of the Spanish in 1769. The Californios lived on vast ranches that had been granted by the Mexican government.
After two years of fighting, the United States emerged the victor. On February 2, 1848, the Treaty of Guadelupe Hidalgo was signed, formally ending the war and handing control of California to the United States. Neither side knew that gold had recently been discovered at the sawmill Swiss immigrant John Sutter was building near Coloma.
When news of gold reached San Francisco first, it was met with disbelief. Then entrepreneur Sam Brannan marched through town waving a vial of the precious metal as proof. By mid-June, stores stood empty. Most of the male population of San Francisco had gone to the mines. The rest of California soon followed. That summer, men like Antonio Franco Coronel, of Los Angeles, dug for gold along side other Californios, Native Americans, and a few Anglo Americans already in California.
Military governor Colonel Richard B. Mason, who toured the gold fields, wrote a report that contained astounding facts: two miners on Weber Creek gathered $17,000 in gold in seven days; six miners with 50 Indians took out 273 pounds of gold; sales at Sam Brannan's merchandise store near the mines totaled $36,000 in May, June and early July. Mason sent his report and a tin of gold to Washington, a trip of many months.