The use of the 49th parallel as a border was not the result of a single event but a series of 19th-century treaties:
Following the War of 1812, Britain and the U.S. agreed to set the 49th parallel as the border from the Lake of the Woods in the east to the Rocky Mountains in the west.
A notable exception occurs at Vancouver Island, where the border swings south to ensure the entire island remains British. Geographic and Cultural Impact
After years of "joint occupancy" and the American political slogan "Fifty-Four Forty or Fight!" (referring to a more northern line), the two nations compromised by extending the 49th-parallel boundary all the way to the Pacific Ocean.