The Iraqi Popular Mobilization Forces (PMF) played a critical role in the fight against the Islamic State. But this paramilitary force now represents a challenge to civil-military relations because many PMF fighters remain loyal to their former commanders despite being formally integrated into the Iraqi armed forces in 2016. Furthermore, because many of these militias retain ties to Iran, the PMF's integration into Iraq's security services provides Iran with a new opportunity to undermine U.S. interests and subvert Iraq's struggling democratic government. Addressing the challenges posed by the PMF will be essential to the success of any U.S. strategy in Iraq and the broader Middle East. In this report, the authors examine lessons learned from previous efforts toward disarmament, demobilization, and reintegration (DDR) of former combatants and militia forces to inform U.S. government policy on the PMF. They analyze historic DDR programming to assess whether a U.S.-supported DDR effort in Iraq might succeed, examining internationally supported DDR programs from 1979 to 2010. This historical analysis indicates that DDR that is focused on the PMF will be extremely difficult unless linked to complementary security sector reform and political reforms that provide greater inclusion. While reducing Iranian-backed PMF power and influence might be beneficial to the United States, a policy that seeks to force that reduction might, in fact, have the opposite effect.