One of the most important and memorable events of the United States" westward push across the frontier came with the discovery of gold in the lands that became California in January 1848. Located thousands of miles away from the country's power centers on the east coast at the time, the announcement came a month before the Mexican-American War had ended, and among the very few Americans that were near the region at the time, many of them were Army soldiers who were participating in the war and garrisoned there. San Francisco was still best known for being a Spanish military and missionary outpost during the colonial era, and only a few hundred called it home. Mexico's independence, and its possession of those lands, had come only a generation earlier.

Everything changed almost literally overnight. While the Mexican-American War technically concluded with a treaty in February 1848, the announcement brought an influx of an estimated 90,000 "Forty-Niners" to the region in 1849, hailing from other parts of America and even as far away as Asia. All told, an estimated 300,000 people would come to California over the next few years, as men dangerously trekked thousands of miles in hopes of making a fortune, and in a span of months, San Francisco's population exploded, making it one of the first mining boomtowns to truly spring up in the West. This was a pattern that would repeat itself across the West anytime a mineral discovery was made, from the Southwest and Tombstone to the Dakotas and Deadwood. Of course, that was made possible by the collective memory of the original California gold rush.