For once I agree with most of what you say, and sweet that it so nicely supports what I said above.

When it took down the Hussein regime, the US didn't destroy a Sunni government. They destroyed a Ba'athist government, which happened to have many Sunnis in it. (The Saudis hate Ba'athists almost as much as they hate Shi-ites.) Having parachuted democracy into Iraq, the USA has no choice but to work with the Shi-ite controlled government of Iraq, which in turn is aligned with Iran. Given its 'druthers the US would much prefer that Sunnis ran Iraq. Second best is that the US can use the Sunnis to keep the Iraqi Shi-ites from acting in total concert with Iran.

IMO, if Islamic State hadn't turned so extremist the US would've happily let them remain in NE Iraq and push the Syrian government into the Mediterranean. But IS turned to extremism, attacking the Kurds and European allies as well. It took a long time, but eventually the US had to act against them.