Second, the chaos threatens Iraq’s stability. Tehran responded to the strike on General Suleimani with missile attacks on two American-run military bases last week. But it’s unlikely this will be the end of Iran’s retaliation. Iranian military strategy is defined by asymmetry — and particularly by the use of militant proxies. Under a screen of plausible deniability, Iran will most likely work to drive the United States out of Iraq.

In this, the Iranians will be brutal. During the American occupation — before the rise of the Islamic State made strange bedfellows of Washington and Tehran — Iranian proxies often exceeded Sunni extremists in terms of the number of casualties they inflicted on American forces. These proxies have lost no time in returning to attacks on American interests. On Sunday, four days after Iran’s missile strikes, a rocket attack on another installation that houses American forces wounded four Iraqi service members.

A conflict between Iranian proxies and the United States will tear at Iraq’s fragile governing structures, creating a power vacuum for the Islamic State to exploit.