Iranian intervention in Iraq (2014–present)

The Iranian intervention in Iraq has its roots in the 2003 invasion of Iraq by the United States and its allies, when the infrastructure of the Iraqi armed forces, as well as intelligence, were disbanded in a process called "de-Ba'athification" which allowed militias with close ties to Tehran to join the newly reconstituted army. The intervention reached its peak following the advance of the Islamic State into northern Iraq in mid-2014.

Source: Wikipedia — Iranian intervention in Iraq (2014–present) (CC BY-SA 4.0)