The United States and Great Britain signed the Treaty of Oregon on June 15, 1846, ending 28 years of joint occupancy of the Pacific Northwest. The treaty established the 49th parallel as the border between the two countries. The United States and Great Britain ended the War of 1812 with the Treaty of Ghent in 1814, and four years later agreed to a 10-year period of joint occupancy of the Northwest.
The joint-occupancy agreement was renewed twice, but by 1846 it was clear that the Northwest was rapidly becoming American, primarily as the result of westward migration on the Oregon Trail, and neither country wanted to fight another war over the border issue. Some members of Congress wanted the northern United States border established at the southern extent of Russian Alaska, at 54 degrees 40 minutes north latitude. Britain would have preferred the Columbia River as the boundary but was willing to consider a border farther north — but not 54 degrees north — and President James Polk, facing war with Mexico, was willing to compromise in order to resolve the issue.
The result of negotiations was a border at 49 degrees north, which gave the British possession of Vancouver Island, an important acquisition for Britain and a concession for the United States. On August 14, 1848, Congress formally established the Oregon Territory, which embraced the present-day states of Washington, Oregon, and Idaho.