A detailed assessment of Islamic State fighting capabilities was provided by the American defense official, who could not be identified under the ground rules for the Pentagon briefing.
The overall picture the official painted of the Islamic State was of an organization losing significant amounts of territory but still determined to fight. All told, the militant group is estimated to have at least 15,000 fighters in Iraq and Syria, although its flow of volunteers has shrunk to 100 a month or even fewer.
No more than 2,500 fighters are struggling to defend western Mosul and the nearby town of Tal Afar, where they are largely cut off by Iraqi forces from receiving supplies and reinforcements. Those fighters have used crude chemical weapons, including chlorine and low-grade sulfur mustard. But while the weapons have hurt civilians, American officials do not believe that they have been militarily effective.
The American-led command has concentrated on striking midlevel commanders to try to sow confusion in the Islamic State’s ranks. The official said the group had lost 65 percent of the terrain that it controlled in Syria and Iraq at its peak.
Still, the official acknowledged, the Islamic State has the resources to carry on, including an abundant supply of weapons and cash that it stole from the Iraqi and Syrian authorities when the militant group swept across the countries in 2014. The resilience of Abu Bakr al-Baghdadi, the group’s leader, may also be a rallying factor. The American defense official said he was believed to be alive and to have left Mosul.
American officials appear confident that the Islamic State will be evicted from Iraq’s major cities and towns within six months. One militant stronghold that the Iraqi government has yet to focus on is Hawija, west of Kirkuk. Iraqi troops bypassed the town, where the defense official said the Islamic State has as many as 1,000 fighters, in the rush to reach Mosul. In October, the militant group used the Hawija area as a springboard for a counterattack on Kirkuk, which was put down after a couple of days of bitter fighting.
The Islamic State has suffered the loss of Al Bab in northern Syria, which fell to Turkish forces and Turkish-backed Syrian militias on Feb. 23. The militants had used the town as a command center and relied on the network of roads there to distribute fighters and supplies.
Within the past week, Palmyra has also been recaptured by the Syrian government, with the help of Russian air power.
But the Islamic State is expected to fight hard in Raqqa, where it has fortified its positions with trenches, tunnels, minefields and houses rigged with explosives.
Raqqa, however, is not the whole story. Thousands of militants are still operating southwest of the city in oil-rich Deir al-Zour, and along the Euphrates in Iraq. The Syrian towns Mayadeen and Abu Kamal are still Islamic State strongholds, as is Qaim across the border in Iraq.
Far from abandoning their dream of a caliphate, Islamic State leaders appear to be making arrangements so that if Raqqa falls, some administrative functions can continue to be carried out in their remaining territory, which the group controls with an iron fist.
The Trump administration has yet to make clear whether it will arm Syrian Kurds for the fight to take Raqqa. Turkey objects to the prospect of assisting the Kurdish minority that straddles the border between the two countries. Another crucial decision is which Syrian groups will govern and secure the city once it is retaken.
One thing is clear, however. Retaking Mosul, Iraq’s second-largest city, would be a body blow to the Islamic State. And capturing Raqqa, a powerful symbol of its power, would be an enormous setback for the group. But more fighting would lie beyond, and the United States and its coalition will need a plan to deal with that.