The borders of Washington State were established as part of the Oregon Territory, which was created in 1848. When the Oregon Territory was divided into several territories, Washington Territory was established in 1853. The borders of Washington have remained relatively stable since then, following its statehood in 1889. Key historical events related to the formation of Washington Territory and its borders include the earlier joint control by Britain and the US in the Pacific Northwest before 1846, the organization of the territory into distinct regions, and the eventual creation of Idaho Territory in 1863, which further defined the area's borders.
Olympia was selected as the capital of Washington Territory in 1853 and has remained a significant political center ever since. Thus, the borders of Washington are largely consistent with those established during the creation of Washington Territory in the mid-19th century.