On August 9, 1842, the United States and Canada signed the Webster–Ashburton Treaty, resolving several border issues between the countries. At the time, Canada was one of the British North American colonies.

Canada and the U.S. had a long-running dispute over borders, particularly the location of the Maine–New Brunswick border. An unofficial war, called the Aroostook War, had broken out in the area between the U.S. and the Canadian provinces of Quebec and New Brunswick. The Webster-Ashtonbury treaty marked the end of the unofficial fighting by formally establishing the border in this area. Unfortunately, the establishment of the border saw the homeland of the Brayon and Native people split between the American state of Maine and the British colony of New Brunswick.

The treaty also established the details of the border between Lake Superior and the Lake of the Woods and reaffirmed the location of the border in the westward frontier up to the Rocky Mountains.

Finally, the treaty called for an end to the slave trade on the high seas, and agreed on terms for shared use of the Great Lakes.

After ten months of negotiation, the treaty was signed on August 9, 1842 by United States Secretary of State Daniel Webster and United Kingdom Privy Counsellor Alexander Baring. A plaque commemorating the treaty was placed at the site of the old State Department building in Washington, D.C. where the signing occurred.