I'm not trying to start another debate on the War of 1812, but I think that everyone can agree that the US could have come away much worse.
Let's say that the US loses most of New England. The North is weakened. The US-Mexican War happens as OTL. Say that the Civil War starts as OTL. But now the Union is weaker without NE and it is weakened enough for the South to be able to win.
America is smaller, weaker, and divided up. What would happen next(Another War with the South? Complete collaspe? More militarism? Less immigration?)?