The United States acquired the area of Washington through a treaty with Great Britain in 1846. Washington Territory was organized from part of Oregon Territory on March 2, 1853; it included all of present-day Washington, northern Idaho, and northwestern Montana. In 1859, when Oregon was admitted as a state, the remainder of Oregon Territory the rest of Idaho, additional area of Montana, and part of Wyoming was added to Washington Territory. Washington Territory assumed generally the same boundary as the present state when Idaho Territory was organized in 1863. Washington was admitted to the Union on November 11, 1889, as the 42nd state.