What were the main overland trails to the West, and how did they
affect the region?

In the American Old West, overland trails were built by pioneers and immigrants throughout the 19th century and especially between 1829 and 1870 as an alternative to sea and railroad transport. These immigrants began to settle much of North America west of the Great Plains as part of the mass overland migrations of the mid-19th century. Settlers emigrating from the eastern United States were spurred by various motives, among them religious persecution and economic incentives, to move to destinations in the far west via routes including the Oregon Trail, California Trail, and Mormon Trail. After the end of the Mexican–American War in 1849, vast new American conquests again enticed mass immigration. Legislation like the Donation Land Claim Act and significant events like the California Gold Rush further lured people to travel overland to the west.

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, Mormon, and California 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 Trail, the Southern Emigrant Trail, and the Old Spanish Trail, 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 infectious diseases, dehydration, malnutrition, injury, and harsh weather, with as many as one in ten travelers dying along the way, usually as a result of disease.

The history of these trails and the settlers who traveled them have since become deeply embedded in the culture and folklore of the United States as some of the most significant influences to shape the content and character of the nation. The remains of many trail ruts can still be observed in various locations throughout the American West. Travelers may loosely follow various routes of the emigrant trails on modern highways through the use of byway signs across the western states.