The event leading Oregon Territory to becoming part of the U.S. was the Oregon Treaty of 1846. This treaty was signed between the United States and Britain, which had joint occupation of the Oregon Territory at the time. The treaty established the 49th parallel as the border between the United States and British North America (now Canada) from the Rocky Mountains to the Pacific Ocean, effectively dividing the territory. Previously, both countries had claimed the entire region of Oregon, but with the signing of the treaty, the southern portion became part of the U.S. while the northern portion became part of British North America.
