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 continental United States. The term "American Old West" refers to a vast geographical area and lengthy time period of imprecise boundaries, and historians' definitions vary. The events in this timeline occurred primarily in the portion of the modern continental United States west of the Mississippi River, and mostly in the period between the Louisiana Purchase in 1803 and the admission of the last western territories as states in 1912 where most of the frontier was already settled and became urbanized; a few typical frontier episodes happened after that, such as the admission of Alaska into the Union in 1959. A brief section summarizing early exploration and settlement prior to 1803 is included to provide a foundation for later developments. Rarely, events significant to the history of the West but which occurred within the modern boundaries of Canada and Mexico are included as well.