The Northwest Boundary Dispute was an argument between the United States and Great Britain over the location of the U.S.-Canada border. The original boundary was established in the Treaty of Paris of 1783, which placed the border as the line extending westwards from the Lake of the Woods' northwestermost point to the source of the Mississippi River. Unfortunately, it was revealed that the map used in these negotiations was faulty, for the source of the Mississippi was actually further south than the treaty indicated. The dispute continued for 26 years, until the United States and Great Britain agreed to the Convention of 1818. This placed the border at the 49th parallel, which is where the international border is today. However, the issue wasn't over, because the Convention only made provisions for the border as far west as the Rocky Mountains; the Oregon Territory was still in dispute.
At first, the U.S. offered to just extend the line to the Pacific Ocean; thereby splitting the Oregon Territory in two. Britain balked at this offer, because the plan would give control of the Columbia River to the United States. Unable to strike a deal, both countries agreed that the Oregon Territory would be occupied by both countries for ten years. When this agreement expired, both nations renewed the treaty for an indefinite length of time. During the Tyler administration, the British gave a new offer to the United States: extend the 49th parallel until it intersected the Columbia, and then use the Columbia itself as the boundary. However, at this time, the United States was in the grip of Manifest Destiny, so the government declined. Not only did the citizens dislike Britain's proposal, they wanted all of the Oregon Territory. This brought about the popular slogan "Fifty-four forty or fight." This referred to the northernmost latitude of the Oregon Territory. It wasn't until the presidency of James Knox Polk that the issue was finally settled. In 1846, the Oregon Boundary Treaty was signed, which essentially instituted the original proposal of the United States: from the 49th parallel all the way to the Pacific. The exact treaty appears below.
From the point on the forty-ninth parallel of north latitude, where the boundary laid down in existing treaties and conventions between the United States and Great Britain terminates, the line of boundary between the territories of the United States and those of her Britannic Majesty shall be continued westward along the said forty-ninth parallel of north latitude 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: Provided, however, That the navigation of the whole of the said channel and straits, south of the forty-ninth parallel of north latitude, remain free and open to both parties.
From the point at which the forty-ninth parallel of north latitude shall be found to intersect the great northern branch of the Columbia River, the navigation of the said branch shall be free and open to the Hudson's Bay Company, and to all British subjects trading with the same, to the point where the said branch meets the main stream of the Columbia, and thence down the said main stream to the ocean, with free access into and through the said river or rivers, it being understood that all the usual portages along the line thus described shall, in like manner, be free and open. In navigating the said river or rivers, British subjects, with their goods and produce, shall be treated on the same footing as citizens of the United States; it being, however, always understood that nothing in this article shall be construed as preventing, or intended to prevent, the government of the United States from making any regulations respecting the navigation of the said river or rivers not inconsistent with the present treaty.
In the future appropriation of the territory south of the forty-ninth parallel of north latitude, as provided in the first article of this treaty, the possessory rights of the Hudson's Bay Company, and of all British subjects who may be already in the occupation of land or other property lawfully acquired within the said territory, shall be respected.
The farms, lands, and other property of every description, belonging to the Puget's Sound Agricultural Company, on the north side of the Columbia River, shall be confirmed to the said company. In case, however, the situation of those farms and lands should be considered by the United States to be of public and political importance, and the United States government should signify a desire to obtain possession of the whole, or of any part thereof, the property so required shall be transferred to the said government, at a proper valuation, to be agreed upon between the parties.
The present treaty shall be ratified by the President of the United States, by and with the advice and consent of the Senate thereof, and by her Britannic Majesty; and the ratifications shall he exchanged at London, at the expiration of six months from the date hereof, or sooner, if possible.