Complex sentence: The Rush-Bagot Treaty signed in 1817 between the United States and the United Kingdom demilitarized the Great Lakes and Lake Champlain, where many British naval armaments and forts still remained, and laid the basis for a demilitarized boundary between the US and British North America.
Split into simpler sentences: The Rush-Bagot Treaty was a treaty conducted between the United States and the United Kingdom in 1817. The treaty provided for the demilitarization of the Great Lakes and Lake Champlain, where many British naval armaments and forts still remained, and laid the basis for a demilitarized boundary between the U.S. and British North America.