You are correct that Al-Maliki was a divisive sectarian leader but Bush could hardly force an overturn of the election of Al-Maliki while he was preaching to the Iraqis about the virtues of democracy.  All he could do was provide sufficient US troops to keep the lid on tribal animosities and on Al Qaeda in Iraq until the next election chose a more capable and less corrupt and sectarian leader.  He did exactly that.  But Obama was antsy to keep his rash political promise to get out of Iraq and he did so against the advice and the pleas of almost all of his generals in the field.  We know how this worked out.  ISIS and other terrorist groups are in Obama's debt.  Others in the ME not so much.

The US kept troops in South Korea and in Germany and should have kept them in Iraq for one overriding reason:  to ensure that US blood and treasure expended in these wars was not squandered away by new conflicts whether by invasion or civil war and to cultivate these nations as strategic partners.