::1855 in the American Old West



1864 Dower hand-colored map depicting the western half of the United States including most of the lands west of the Mississippi River.

This timeline of the American Old West is a chronologically ordered list of events significant to the development of the American West as a region of the United States prior to 1912. The area, generally defined as lands west of the Mississippi River, was inhabited for millennia by various groups of Native Americans. While the West was a homeland to many, it served as a frontier to European powers. The Spanish began colonization in the 16th century in the southern parts of what is now the American West. The British, French, and Russians attempted colonization at varying levels for extended periods of time. From the 18th century forward, the newly independent United States began securing its own frontier from the Appalachian Mountains westward for settlement and economic investment by American citizens. The long history of American expansion into these lands has played a central role in shaping American culture, iconography, and the modern national identity, and remains a popular topic for study by scholars and historians.

The events in this timeline occurred in the contiguous portion of the modern United States west of the Mississippi River, focused on the period roughly between the Louisiana Purchase in 1803 and the admission of the last mainland states into the Union in 1912. A small section summarizing early exploration and settlement prior to 1803 is included to provide a foundation for later developments. Rarely, events occurring outside the American West, such as in Canada or Mexico, are included due to a significant relation to events in the West. Events listed below are notable developments for the region as a whole, not just for a particular state or smaller subdivision of the region. They represent or illustrate the development of the American West as a frontier, as historians Hine and Faragher put it, "tell[ing] the story of the creation and defense of communities, the use of the lands, the development of markets, and the formation of states.... It is a tale of conquest, but also one of survival, persistence, and the merging of peoples and cultures."<ref>{{#invoke:citation/CS1|citation |CitationClass=book }}</ref>

1855 in the American Old West sections