Question

How did this map change following the treaty of Washington in 1846

Answer 1

The U.S. and Great Britain fought over the border between the U.S. and Canada. In 1818, they had agreed that the Oregon Territory would be settled by both countries. In 1846, though, the Treaty of Washington expanded the U.S. border with Canada at the 49th parallel to the Pacific Ocean.
Oregon Country was divided between Britain and the United States at the 49th Parallel.