U.S., Great Britain sign Oregon Treaty, June 15, 1846

On this day in 1846, the United States and Great Britain averted waging a potential third war in 70 years by signing the Oregon Treaty in Washington. It established the 49th parallel, from the Rocky Mountains to the “major channel” running through Strait of Juan de Fuca, which flows into the Pacific Ocean, as the boundary between the United States and Canada.

The treaty was negotiated by Secretary of State James Buchanan, who would become the nation’s 15th president, and Sir Richard Pakenham, the British minister to the United States. Under the accord, the United States gained undisputed control over what became the future states of Washington, Oregon, Idaho and Montana while dropping its claims to the future Canadian provinces of British Columbia and Alberta.

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Under an 1818 pact, the border ran westward astride the 49th parallel from Lake of the Woods in present-day Minnesota to the Rocky Mountains. That agreement called for joint control of the Oregon territory for 10 years, an arrangement that was extended for another 10 years in 1827.

In 1844, with the territory’s boundary still unsettled, Democrat James Polk won the presidency on the slogan of “Fifty-four forty or fight,” the southern limit of Russian America — now Alaska. Such a boundary would have brought much of what is now westernmost Canada into the United States. However, with new war with Mexico occupying Polk, a compromise with Britain over the disputed Oregon territory proved providential.

The British side of the border remained unorganized until 1858 when the colonies of Vancouver Island and British Columbia were set up amid fears of reasserted U.S. expansionism. They were amalgamated in 1866. When they joined Canada in 1871, the 49th parallel, as demarcated by the Oregon Treaty, remained the U.S.-Canadian border, until it zigzags southward through the strait and empties into the Pacific Ocean.