On October 20, 1803, the United States Senate ratified the Louisiana Purchase.
Exactly 15 years later, the Convention of 1818 signed between the United States and the United Kingdom which, among other things, settled the Canada-United States border on the 49th parallel for most of its length.
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In the struggle amongst the United States and the Empires of Britain, Spain, and Russia over the Pacific Northwest, the extension of that imaginary line to the west of the Rocky Mountains had to wait.