I know very well that the US backed Saddam in his war against Iran and sold him chemical and other weapons. I also know they ended up supplying Iran with missiles (contra Affair).
The dirty fingers of the US and in particular the CIA have been and are still in every conflict in this world since WW2.
However back to ISIS;
It would seem that once again the US is fighting their own weapons for they appear to be the major supplier of ISIS by default.
IS conflict: How is it getting hold of weapons from the West? - BBC News
https://www.amnesty.org/en/latest/ne...rms-transfers/
Without this ready supply of weapons and the inclusion of the remains of Saddam's army ISIS would not be the threat to the world that it is today.
Had Saddam been left in power in all probability he would have wiped or chased out Zarqawi and his group.
As you would have seen if you had read the link I posted earlier Assad is making peace with the rebels and giving them some autonomy now their weapons supply has been curtailed and making inroads into ISIS which as it gets defeated in Syria and Iraq is taking its terror into the countries that it sees as enemies.