Retired General David Petraeus, who commanded U.S. troops during the 2007-2008 surge, was back in Iraq last week for the first time in more than three years.

In his most expansive comments yet on the latest crisis in Iraq and Syria, he answered written questions from The Washington Post’s Liz Sly:

I would argue that the foremost threat to Iraq’s long-term stability and the broader regional equilibrium is not the Islamic State; rather, it is Shiite militias, many backed by — and some guided by — Iran.

These militia returned to the streets of Iraq in response to a fatwa by Shia leader Grand Ayatollah Sistani at a moment of extreme danger. And they prevented the Islamic State from continuing its offensive into Baghdad. Nonetheless, they have, in some cases, cleared not only Sunni extremists but also Sunni civilians and committed atrocities against them. Thus, they have, to a degree, been both part of Iraq’s salvation but also the most serious threat to the all-important effort of once again getting the Sunni Arab population in Iraq to feel that it has a stake in the success of Iraq rather than a stake in its failure. Longer term, Iranian-backed Shia militia could emerge as the preeminent power in the country, one that is outside the control of the government and instead answerable to Tehran.

Beyond Iraq, I am also profoundly worried about the continuing meltdown of Syria, which is a geopolitical Chernobyl. Until it is capped, it is going to continue to spew radioactive instability and extremist ideology over the entire region.

Any strategy to stabilize the region thus needs to take into account the challenges in both Iraq and Syria. It is not sufficient to say that we’ll figure them out later.

What went wrong in Iraq?

There was certainly a sense in Washington that Iraq should be put in our rearview mirror, that whatever happened here was somewhat peripheral to our national security and that we could afford to redirect our attention to more important challenges. Much of this sentiment was very understandable given the enormous cost of our efforts in Iraq and the endless frustrations that our endeavor here encountered.

In retrospect, a similar attitude existed with respect to the civil war in Syria — again, a sense that developments in Syria constituted a horrible tragedy to be sure, but a tragedy at the outset, at least, that did not seem to pose a threat to our national security.

But in hindsight, few, I suspect, would contend that our approach was what it might — or should — have been. In fact, if there is one lesson that I hope we’ve learned from the past few years, it is that there is a linkage between the internal conditions of countries in the Middle East and our own vital security interests.

The current Iranian regime is not our ally in the Middle East. It is ultimately part of the problem, not the solution. The more the Iranians are seen to be dominating the region, the more it is going to inflame Sunni radicalism and fuel the rise of groups like the Islamic State. While the U.S. and Iran may have convergent interests in the defeat of Daesh, our interests generally diverge. The Iranian response to the open hand offered by the U.S. has not been encouraging.

Iranian power in the Middle East is thus a double problem. It is foremost problematic because it is deeply hostile to us and our friends. But it is also dangerous because, the more it is felt, the more it sets off reactions that are also harmful to our interests — Sunni radicalism and, if we aren’t careful, the prospect of nuclear proliferation as well.

You have had some interactions with Qassem Soleimani in the past. Could you tell us about those?

In the spring of 2008, Iraqi and coalition forces engaged in what emerged as a decisive battle between the Iraqi Security Forces and the Iranian-supported Shiite militias.

In the midst of the fight, I received word from a very senior Iraqi official that Qassem Soleimani had given him a message for me. When I met with the senior Iraqi, he conveyed the message: “General Petraeus, you should be aware that I, Qassem Soleimani, control Iran’s policy for Iraq, Syria, Lebanon, Gaza, and Afghanistan.” The point was clear: He owned the policy and the region, and I should deal with him.