U.S.-Canadian border established - Jun 15, 1846 - HISTORY.com

Representatives of Great Britain and the United States sign the Oregon Treaty, which settles a long-standing dispute with Britain over who controlled the Oregon territory. The treaty established the 49th parallel from the Rocky Mountains to the Strait of Georgia as the boundary between the United States and British Canada. The United States gained formal control over the future states of Oregon, Washington, Idaho, and Montana, and the British retained Vancouver Island and navigation rights to part of the Columbia River.

In 1818, a U.S.-British agreement had established the border along the 49th parallel from Lake of the Woods in the east to the Rocky Mountains in the west. The two nations also agreed to a joint occupation of Oregon territory for 10 years, an arrangement that was extended for an additional 10 years in 1827. After 1838, the issue of who possessed Oregon became increasingly controversial, especially when mass American migration along the Oregon Trail began in the early 1840s.

American expansionists urged seizure of Oregon, and in 1844 Democrat James K. Polk successfully ran for president under the platform “Fifty-four forty or fight,” which referred to his hope of bringing a sizable portion of present-day Vancouver and Alberta into the United States. However, neither President Polk nor the British government wanted a third Anglo-American war, and on June 15, 1846, the Oregon Treaty, a compromise, was signed. By the terms of the agreement, the U.S. and Canadian border was extended west along the 49th parallel to the Strait of Georgia, just short of the Pacific Ocean.