The Frankfurt proposals was a peace initiative by Austrian minister Metternich offered to Emperor Napoleon in early 1813 after he had suffered serious military defeats. Napoleon would remain as Emperor of France, but it would be reduced to its "natural frontiers." That meant that France could retain control of Belgium, Savoy and the west bank of the Rhine River, while giving up control of all the rest.

Metternich's motivation was to maintain France as a balance against Russian threats, while ending the highly destabilizing series of wars.J. P. Riley (2013). Napoleon and the World War of 1813: Lessons in Coalition Warfighting. Routledge. p. 206.

Napoleon, expecting to win the war, rejected the Frankfurt proposals. When the Allies invaded France in 1814 and Napoleon was heavily outnumbered, he tried to reopen peace negotiations on the basis of accepting the Frankfurt proposals. The Allies now had new, harsher terms that included the retreat of France to its 1791 boundaries, which meant the loss of Belgium. Napoleon adamantly refused.