How did this map change following the Treaty of Washington in 1846? The Oregon Country was divided between Britain and the United States at the 49th Parallel.
The Oregon Country was split into the Washington and Oregon Territories using the 49th Parallel as the new boundary.
The boundary between the Oregon Country and Mexico was shifted from the 42nd Parallel to the Oregon Trail route.
The portion of the Oregon Country below the 49th Parallel became part of Mexico, while the portion above the 49th Parallel became part of Great Britain.