The document discusses American expansion and settlement in the trans-Mississippi West between 1815-1848. It describes how Spanish and Mexican control of the region gave way to American annexation of Texas and conquest of territory in the Mexican-American War. The ideology of Manifest Destiny and the Oregon dispute with Britain also contributed to U.S. territorial growth. Settlers moved West for economic opportunities like fur trading, land speculation, and the California Gold Rush. Their arrival introduced conflicts with Native peoples over resources and cultural differences.