Anglo-American Convention sets the 49th parallel boundary

Background. Prior treaties, including the Treaty of Paris (1783) and the Treaty of Ghent (1814), failed to accurately define the boundary near the Lake of the Woods. The 1783 treaty described a line running through the Lake of the Woods to its northwesternmost point and thence due west to the Mississippi River, a course that proved impossible. By 1817, settlers were moving west of the lake, and both sides saw a need to settle the boundary west of the 1783 line. The 1818 Convention corrected these errors by running the boundary due north or south from the lake's northwest point to the 49th parallel.

Geographical Impact. The treaty created the Northwest Angle, a pene-exclave of Minnesota, as the only part of the contiguous United States north of the 49th parallel. This resulted from negotiators misunderstanding the geography of the Lake of the Woods. The Angle is separated from the rest of Minnesota by the lake but shares a land border with Manitoba. The boundary was fixed by running due west along the 49th parallel from the lake.

Diplomatic Context. The Convention of 1818 marked improved relations between the British Empire and the United States, alongside the Rush–Bagot Treaty. It allowed for joint occupation of the Oregon Country. Article IV confirmed the 1815 commercial convention, and Article V referred slave compensation claims to a friendly sovereign. Ratifications were exchanged in January 1819.