During the colonial period, various claims to the Pacific Northwest were made by Russia, Spain, and the United Kingdom. The United States established a claim following the exploration of the region by the Lewis and Clark Expedition. From the 1810s until the 1840s, modern-day Washington, Oregon, Idaho, and western Montana, along with most of British Columbia, were part of the Oregon Country, which was jointly administered by the United States and the United Kingdom after the Treaty of 1818. John McLoughlin of the Hudson's Bay Company was the de facto local political authority for most of this time.
This arrangement ended as U.S. settlement grew and Polk was elected on a platform of "Fifty-Four Forty or Fight"; after a war scare with the United Kingdom, the two nations negotiated the 1846 Oregon Treaty, partitioning the region along the 49th parallel and resolving most (but not all) of the border disputes.
British Columbia, the UK portion, joined the Dominion of Canada in 1871. The U.S. portion became the Oregon Territory; it was later subdivided into territories that were eventually admitted as states.
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