The Webster-Ashburton Treaty, signed August 9, 1842, settled the dispute over the location of the Maine-New Brunswick border between the United States and Canada, then a colony of Britain. It also established the details of the border between Lake Superior and the Lake of the Woods, originally defined in the Treaty of Paris (1783); reaffirmed the location of the border (at the 49th parallel) in the westward frontier up to the Rocky Mountains, originally defined in the Treaty of 1818; called for a final end to the slave trade on the high seas, to be enforced by both signatories; and agreed on terms for shared use of the Great Lakes.
The treaty was signed by United States Secretary of State Daniel Webster and United Kingdom Privy Counsellor Alexander Baring, 1st Baron Ashburton. A plaque commemorating the treaty was placed at the site of the old State Department building in Washington, D.C. where the signing occurred.
The treaty was responsible for a geographic oddity. Since "Fort Blunder," an unnamed U.S. fort in northeastern New York, had been constructed on Canadian soil, the northern borders of New York east of the St. Lawrence and Vermont were adjusted to 3/4 of a mile north of the 45th parallel, thus placing the half finished and abandoned fort on U.S. soil. Following the signing of the treaty, construction was once again begun on the site in 1844, replacing the aborted 1812 era construction with a massive 3rd system masonary fortification known as Fort Montgomery.
This treaty marked the end of unofficial fighting (known informally as the Aroostook or Lumberjack's War) along the Maine-New Brunswick border and resolved issues that had led to the Indian Stream conflict as well as the Caroline Affair. The border was fixed with the disputed territory divided between the two nations. The British acquired the Halifax-Quebec route they desired. Also, as a result of this treaty, portions of the western U.S.-Canada border were adjusted so as to be consistent. It gave the U.S. negligibly more land to the north. The Creole case was passed over by both nations.
The border between Lake Superior and the Lake of the Woods needed clarification because the faulty Mitchell Map used in the negotiations for the Treaty of Paris (1783) was inadequate to define the border according to the terms of that treaty. The ambiguity in the map and treaty resulted in Minnesota's Arrowhead region being disputed, and previous negotiations had not resolved the question. The treaty speaks of the border passing through "Long Lake", the location of which was unstated, but the map showed the lake flowing out into Lake Superior near Isle Royale, which is consistent with the Pigeon River route. The British however had previously taken the position that the border should leave Lake Superior at Fond du Lac (the "head of the lake") near modern Duluth, Minnesota, proceed up the St. Louis and Embarrass Rivers, across the height of land, and down Pike River and Lake Vermilion to the Rainy River. To counter this western route, the U.S. advocated for an eastern route, used by early French explorer Jacques de Noyon in 1688 and the later location of the fur-traders route after 1802. This way headed north from the lake at the site of Fort William, Ontario up the Kaministiquia and Dog Rivers to Cold Water Lake, crossed the divide by Prairie Portage to Height of Land Lake, then went west by way of the Savanne, Pickerel, and Maligne Rivers to Lake La Croix where it joined the present border. The Mitchell map had shown both of those routes, and also showed the "Long Lake" route between them. Long Lake was thought to be the Pigeon River (despite the absence of a lake at its mouth), and the traditional traders' route there left the Lake at Grand Portage and went overland to the Pigeon, up that river and a tributary across the Height of Land Portage, and thence down tributaries of the Rainy River to Lac La Croix, Rainy Lake and River, and Lake of the Woods. This is the route which was designated as the border in the treaty.
Another clarification made in this treaty resulted in cementing the anomaly of the Northwest Angle. Again, due to errors on the Mitchell Map, Treaty of Paris reads "…through the Lake of the Woods to the most northwesternmost point thereof, and from thence on a due west course to the river Mississippi…." With the border clarification established by the Anglo-American Convention of 1818 defining the boundary about Lake of the Woods to the Rocky Mountains, this treaty reaffirmed the border and further detailed the border by modifying the border definition to instead read as "…at the Chaudiere Falls, from which the Commissioners traced the line to the most northwestern point of the Lake of the Woods, thence, along the said line to the said most northwestern point, being in latitude 49°23′55″ north, and in longitude 95°14′38″ west from the Observatory at Greenwich; thence, according to existing treaties, due south to its intersection with the 49th parallel of north latitude, and along that parallel to the Rocky Mountains…."
Ultimately, the only "losers" were the original Brayon (and Native) inhabitants of the region, who saw not only their homeland but also their people split between the American state of Maine and the British colony of New Brunswick.
Shortly after ratification of this treaty, the Ojibwa nations about the south shore of Lake Superior ceded land to the United States in the Treaty of La Pointe. However, the news of the ratification of this treaty did not reach either negotiation party for the second treaty, causing the Grand Portage Band to be omitted for the treaty council. In addition, the Grand Portage Band was misinformed on the details of the Treaty of Paris, thinking the border passed through the center of Lake Superior to the Saint Louis River, placing both Isle Royale and themselves in British territory, though Treaty of Paris specifically mentions Isle Royale to be in the territories of the United States. Consequently, the Isle Royale Agreement was signed between the United States and the Grand Portage Band in 1844 as an adhesion to the Treaty of La Pointe, with other Ojibwa tribes reaffirming the treaty.
Ten months of negotiations for the treaty were held largely at the Ashburton House, home of the British legation on Lafayette Square in Washington, D.C. The house has been designated a U.S. National Historic Landmark.
In order to make the controversial treaty more popular in the United States, Webster released a map of the Maine-Canada border which he claimed had been drawn by Benjamin Franklin. Many historians believe that this map was forged.