On June 15, 1846, Britain and the United States sign the Treaty of Oregon establishing the 49th parallel as the primary international boundary in the Pacific Northwest. Since 1818, the entire region, including what is now Washington, Oregon, and Idaho, and large portions of British Columbia, has been under joint occupancy, in which citizens of both countries travel and trade freely. In 1845, Americans adopt the phrase "Fifty Four Forty or Fight!", referring to the latitude line of 54 degrees and 40 minutes, a more northern border that would give the U.S. a large portion of what is Canada today. However, the U.S. proves unwilling to actually go to war over this issue, because it is facing war with Mexico on the southern border. So the U.S. and Britain compromise by extending the 49th parallel border -- long established east of the Rockies -- all the way to the Pacific. In the only exception to the 49-degree line, both parties agree to allow the border to swing south around Vancouver Island, which gives Britain all of Vancouver Island. The mainland border between the U.S. and Canada will remain unchanged from that time onward.
Both Britain and the U.S. believed they had the prior historical claim to the Columbia River and its vast drainages. The first non-Indian explorer to enter the Columbia's mouth was American Robert Gray (1755-1806) in 1792, who named the river after his ship, the Columbia Rediviva. Several months later, the British Navy's William Broughton (1762-1821), under Captain George Vancouver (1758-1798), rowed 100 miles up the river. Also, British fur trader David Thompson (1770-1857) was the first non-Indian to canoe the river from its sources in the shadow of the Canadian Rockies. Both countries were unwilling to settle on a definitive border agreement, and in 1818 they agreed to joint occupation.
In the ensuing decades, they each maneuvered to establish sovereignty over the region. In 1825, the governor of the Hudson's Bay Company proposed a more southerly boundary that would have given Britain large chunks of Montana and Idaho, almost all of present-day Washington, and the entire Columbia River. The Americans rejected that idea. Meanwhile, the U.S. began encouraging settlers to move to the Oregon Country, which at the time encompassed the entire Northwest.
In 1844, U.S. politicians began clamoring for a definitive answer to the Oregon question. James K. Polk (1795-1849) had just been elected U.S. president and his expansionist supporters had adopted the slogan "Fifty Four Forty or Fight!" (Johansen and Gates, 205). They chose that particular latitude line, 54-40, because it was already established as the southern boundary of Russian Alaska.
As it turned out, the U.S. Senate was not willing to fight for that high northern border. The government preferred to compromise since it was preparing to embark on a different border war: the Mexican War. The British first suggested that the 49-degree line be extended to the Columbia River and then along the river to the sea. This would have given Britain the entire Columbia River, as well as Puget Sound. American negotiators rejected the proposal and countered by suggesting a simple straight line, clear across the continent -- the 49-degree line, straight through Vancouver Island, cutting the island in two.
When Britain proved unwilling to give up its new Fort Victoria on the island, the negotiators compromised on one exception to the 49-degree line: Britain would get all of Vancouver Island. The treaty language specified that the 49-degree line would extend only "to the middle of the channel which separates the continent from Vancouver's Island; and thence southerly through the middle of the said channel; and of Fuca's Straits to the Pacific Ocean" (Johansen and Gates, 207). In other words, the border would curve around the southern end of Vancouver Island. The final treaty was signed on June 15, 1846, and ratified on June 19, 1846.
The treaty proved to be a long-term peaceful solution to this contentious issue, yet it had two unforeseen consequences. The language specifying "the middle of the channel" around Vancouver Island proved to be too vague. There were, in fact, several channels separating Vancouver Island from the continent, with numerous islands, including the San Juan Islands, scattered in the way. Both countries came to believe that they had rights to the San Juan Islands. This later resulted in the seriocomic San Juan Island Pig War in 1859 and an armed standoff between the two countries that lasted until 1872, when the U.S. was awarded the San Juan Islands in arbitration.
The second consequence arose from the fact that the 49th parallel cuts the Columbia River in two. The first 498 miles of river are in Canada, the final 745 miles in the U.S. With the coming of dams on the Columbia beginning in the 1930s, this became an increasingly important issue. In 1961, Canada and the U.S. signed the Columbia River Treaty, ratified in 1964, in which the two countries agreed to a system of joint management of the river's water and power.