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The Territory of Washington was an organized incorporated territory of the United States that existed from March 2, 1853, until November 11, 1889, when the territory was admitted to the Union as the State of Washington. It was created from the portion of the Oregon Territory north of the lower Columbia River and north of the 46th parallel east of the Columbia. At its largest extent, it also included the entirety of modern Idaho and parts of Montana and Wyoming, before attaining its final boundaries in 1863.
Upon the admission of the State of Oregon to the union in 1859, the eastern portions of the Oregon Territory, including southern Idaho, portions of Wyoming west of the continental divide (then Nebraska Territory), and a small portion of present-day Ravalli County, Montana were annexed to the Washington Territory. In 1863, the area of Washington Territory east of the Snake River and the 117th meridian west was reorganized as part of the newly-formed Idaho Territory, leaving that territory with only the lands within the current boundaries of the State of Washington.