The theory that the US government is limited by checks and balances depends on each branch of government wanting power. Unfortunately, this is not the case anymore:
And this, it seems to me, is the real story of presidential power over warmaking. It’s not so much that we have executive branch power grabs as it is that congress pretty consistently fails to use the leverage that it actually has.
I think this is probably a failure of democracy. If Congressmen do nothing to start and stop wars, they don’t have to be responsible for the outcomes. If something bad happens in the war, they can blame the other guy. If something good happens they can take part of the credit. Democracy disincentives responsibility and therefore good government.