The Webster–Ashburton Treaty was a treaty that resolved several border issues between the United States and the British North American colonies (the region that later became the Dominion of Canada). Negotiated in the US federal capital city of Washington, DC, it was signed August 9, 1842, under the new administration of US president John Tyler, who as the former vice president, had just recently succeeded and became chief executive upon the unexpected death of his running mate and predecessor, William Henry Harrison, who had only served a single month in office. The Daniel Webster–Lord Ashburton negotiations and the newly drawn-up 1842 treaty resolved many of the issues arising from the recent border conflicts and skirmishes between Americans and New Brunswickers in the Aroostook War of 1838–1839. It arose from disputes and controversies over the vague indefinite terms and text of the old peace agreement of the Treaty of Paris of 1783, which ended the American Revolutionary War.
The provisions of the 1842 treaty between Britain and the United States included:
The settlement of the location of the Maine–New Brunswick international border, which was the primary cause of the Aroostook War.
Establishment of the international border between Lake Superior and the Lake of the Woods, originally defined in the Treaty of Paris in 1783.
Reaffirmation of the location of the border (at the 49th parallel) in the westward frontier up to the far western Rocky Mountains defined in the previous Treaty of 1818.
Definition of seven crimes subject to extradition.
Agreement that the two parties would share use of the Great Lakes.
Agreement that there should be an end to the slave trade on the high seas.
The treaty also retroactively confirmed the southern boundary of the Province of Quebec that land surveyors John Collins and Thomas Valentine had marked with stone monuments in 1771–1773. The treaty intended that the international border be fixed at the 45 degrees north parallel of latitude. Still, the border is in some places nearly 0.8 km (1⁄2 mi) north of the 45th parallel. The treaty was signed by US secretary of state Daniel Webster, and British diplomat Alexander Baring, 1st Baron Ashburton.
An arbitration of various border issues in the East before King William I of the Netherlands in 1831 had failed to yield a binding decision.
The Treaty of Paris had established the 45th parallel as part of the northern boundary of modern-day New York and Vermont. Most of that portion of the boundary had previously been surveyed in the early 1770s, but the survey line was inaccurate. Since "Fort Blunder"—an unnamed U.S. fort in what is now part of northeastern New York—had been constructed north of the actual 45th parallel, the United States wanted to follow the old survey line, and the Webster–Ashburton treaty incorporated this change, leaving the half-finished fort on U.S. soil. Following the signing of the treaty, the U.S. resumed construction on the site. The new project replaced the aborted 1812-era construction with a massive third-system masonry fortification known as Fort Montgomery.
This treaty marked the end of local confrontations between lumberjacks (known as the Aroostook War) along the Maine border with the British colonies of Lower Canada (which later became Quebec) and New Brunswick. The newly agreed border divided the disputed territory between the two nations. The British were assigned the Halifax–Quebec road route, which their military desired because Lower Canada had no other connection in winter to New Brunswick and Nova Scotia. The treaty adjusted portions of the border to give the United States a little more land to the north. It also resolved issues that had led to the Indian Stream dispute as well as the Caroline Affair. The Indian Stream area was assigned to the United States. The Webster–Ashburton Treaty failed to clarify ownership of Machias Seal Island and nearby North Rock, which remain in dispute. Additionally, the signing of the treaty put an end to several building improvements planned for Upper Canadian defense forts such as Fort Malden in Amherstburg, which the British government later abandoned, as they no longer served a defensive purpose.
The border between Lake Superior and the Lake of the Woods in the Great Lakes region 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 British-American treaty. Ambiguity in the map and treaty resulted in Minnesota's Arrowhead region being disputed between the two nations years later, and previous negotiations had not resolved the question. The treaty had the border pass through Long Lake, but did not state that lake's location. However, the map showed the lake flowing 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") in modern Duluth, Minnesota, proceed up the Saint 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. side advocated for an eastern route, used by early French explorer Jacques de Noyon in 1688, and later a well-used fur traders' route after 1802. This way headed north from the lake at the site of Fort William 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 international 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).
The traditional traders' route 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 finally the route the treaty designated as the border.
The treaty clarified the channel that the border would follow between Lake Huron and Lake Superior, awarding Sugar Island to the U.S. Another clarification made in this treaty resulted in clarifying the anomaly of the Northwest Angle. Again, due to errors on the Mitchell Map, the 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 ..." In fact, a course due west from the Lake of the Woods never intersects the Mississippi. The Anglo-American Convention of 1818 defined the boundary of Lake of the Woods to the Rocky Mountains.
This 1842 treaty reaffirmed the border and further defined it by modifying the border definition to instead read as: