May 21, 2015 5:34 AM EDT

The fall of Ramadi to the Islamic State of Iraq and Greater Syria (ISIS) on May 17 was a body blow to the U.S.-led war against the extremist group. The capital of Iraq’s Anbar province, Ramadi is the biggest prize captured by ISIS since it overran Mosul in a shock offensive last June.

Ramadi’s fall, nine months into a U.S. bombing campaign that the Pentagon had framed as a slow, rolling success, illustrates not only the limits of airpower–ISIS attacked during a dust storm that grounded U.S. planes–but also the continuing weakness of the Iraqi government led by Prime Minister Haider al-Abadi.

“It is a very worrying development,” says Jessica Lewis McFate, research director at the Institute for the Study of War.

Ramadi is where ISIS first gained traction in Iraq. In late 2013, Nouri al-Maliki, al-Abadi’s openly sectarian predecessor, crushed a peaceful gathering of Sunni residents in the city protesting the government’s favoritism toward the majority Shi’ite population. The repression let ISIS cast itself as protector of the Sunnis and assemble tribal allies through much of the Sunni western reaches of Iraq that it now holds.

The U.S.-backed al-Abadi was brought into office last September in part to coax Sunnis back into the Iraqi government. But with the collapse of the Iraqi army in Ramadi, Shi’ite militias, some backed by Iran, rushed to take over the fight for a Sunni city where they have not been welcome. The result could be sectarian bloodshed even if ISIS is eventually ousted from Ramadi.

The defeat has increased calls for President Obama to send U.S. commandos into the fight against ISIS, both to guide air strikes and to advise Iraqi commanders on the ground. The 3,000 U.S. military personnel in Iraq now keep well behind the lines, training Iraqi forces who still appear outmatched. A White House spokesman said Obama may “tweak the strategy” on the ground, but McFate calls for “shredding strategic assumptions.” Either way, it’s quickly becoming clear that ISIS is a tougher enemy than most predicted.

This appears in the June 01, 2015 issue of TIME.

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