In the history of the United States, American pioneers built overland trails throughout the 19th century, especially between 1840 and 1847 as an alternative to sea and railroad transport. These settlers began to settle much of North America west of the Great Plains as part of the overland mass settlements of the mid-19th century. Settlers emigrating from the eastern United States did so with various motives, among them religious persecution and economic incentives, to move from their homes to destinations further west via routes such as the Oregon, California, and Mormon Trails. After the end of the Mexican–American War in 1848, vast new American conquests of territory again encouraged mass settlement. Legislations like the Donation Land Claim Act and significant events like the California Gold Rush further encouraged settlers to travel overland to the north.
Two major wagon-based transportation networks, one typically starting in Missouri and the other in the Mexican province of Santa Fe de Nuevo México, served the majority of settlers during the era of westward expansion. Three of the Missouri-based routes—the Oregon, California, and Mormon Trails—were collectively known as the Emigrant Trails. Historians have estimated at least 500,000 emigrants used these three trails between 1843 and 1869, and despite growing competition from transcontinental railroads, some use even continued into the early 20th century. The major southern routes were the Santa Fe, Southern Emigrant, and Old Spanish Trails, as well as its wagon road successor the Mormon Road, a southern spur of the California Trail used in the winter that also made use of the western half of the Old Spanish Trail. Regardless of the trail used, the journey was often slow and arduous, fraught with risks from dysentry, infectious diseases, dehydration, malnutrition, cholera, highwaymen, Indian raids, injury, and harsh weather, with as many as one in ten travelers dying along the way, usually as a result of disease.