He said the Syrian civil war and the policies of Iraq's former prime minister, Nouri al-Maliki, contributed to the terrorist group flourishing in the region more so than the 2003 US invasion of Iraq or eventual troop drawdown.
"I think the primary reasons for the rise of ISIS are the Syrian civil war and the policies followed by the government in Baghdad," Gates told Business Insider on Monday.
The Syrian civil war certainly is the major factor in allowing ISIL to rise after that fighting dragged on to pull in jihadis from abroad.
Perhaps if we'd actually made an effort to defeat Assad rather than just declare that he had to go while doing nothing to achieve that assumed inevitable outcome, this would have been a revolt and overthrow rather than a sectarian civil war. But it is hard for me to say that an aggressive American-led effort would have worked. We just know that hoping for a better outcome than sectarian civil war, 300,000 dead, the use of chemical weapons, the rise of ISIL to spill into Iraq and the wider region, and Russia's intervention in Syria, did not work.
And Maliki's policies certainly did alienate Sunni Arabs who decided to support ISIL.
But the Maliki who alienated Sunni Arabs is not the Maliki who we backed while we were in Iraq in force prior to our departure in 2011.
That Maliki went after the pro-Iran Shia militias in the spring 2008 Charge of the Knights offensive in the south, centered on Basra.
Our sizable presence meant that Maliki had a counter-weight to Iranian religious-based influence. But without us there, Iranian power lacked a real check, and so Maliki had to alter his policies to favor Shias (and Iran) at the expense of Sunni Arabs just to survive.
On the other side, without our presence in strength, the Sunni Arabs did not have a champion to call on to argue for their side of the argument. Remember, even though we destroyed the Saddam regime in 2003 and ended Sunni Arab minority rule in Iraq that exploited the Shias and Kurds for Sunni Arab benefit, the Sunni Arabs turned to us for help in the 2006-2007 Awakening that led the Sunni Arabs to largely turn against al Qaeda in Iraq (the foundation of ISIL).
As long as we remained in Iraq in strength to defend rule of law, the Sunni Arabs could appeal to us to prevent the Iraqi government (dominated by the Shias, as their numbers justify) from treating them unfairly. When we left, the only power to appeal to was the still-surviving jihadi movement in Iraq.
And without our sizable presence in Iraq after 2011, the efforts we made to create an effective Iraqi military ended and the resulting corruption and sectarian policies undermined the ability of the Iraqi army to fight, meaning that Iraqi forces did not finish off al Qaeda in Iraq. Instead, the Iraqis allowed the remnant jihadis who survived the Surge offensive/Awakening to rebuild and eventually eject the Iraqi security forces from northern Iraq in June 2014 (and from western Iraq in January 2014).
And of course, don't forget that the ultimate underlying cause for the rise of ISIL is the unfortunate tendency of Islam to provide holy justifications for jihadi murder sprees. Islam has to put their house in order to delegitimize the jihadi impulse.
That's why this is a long war and ISIL is just the most prominent enemy at the moment.