The Webster–Ashburton Treaty is signed, establishing the United States–Canada border east of the Rocky Mountains.

The Webster–Ashburton Treaty, signed August 9, 1842, was a treaty that resolved several border issues between the United States and the British North American colonies. 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 newly drawn-up 1842 treaty resolved many of the issues of 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 Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch 3,000 miles in straight-line distance from the northernmost part of Western Canada, to New Mexico in the Southwestern United States. Depending on differing definitions between Canada and the U.S., its northern terminus is located either in northern British Columbia's Terminal Range south of the Liard River and east of the Trench, or in the northeastern foothills of the Brooks Range/British Mountains that face the Beaufort Sea coasts between the Canning River and the Firth River across the Alaska-Yukon border. Its southernmost point is near the Albuquerque area adjacent to the Rio Grande rift and north of the Sandia–Manzano Mountain Range. Being the easternmost portion of the North American Cordillera, the Rockies are distinct from the tectonically younger Cascade Range and Sierra Nevada, which both lie farther to its west.