BAGHDAD >> The Islamic State’s latest suicide attack in Baghdad, which killed nearly 330 people, foreshadows a long and bloody insurgency, according to U.S. diplomats and commanders, as the group reverts to its guerrilla roots because its territory is shrinking in Iraq and Syria.

Officials say many Islamic State fighters who lost battles in Fallujah and Ramadi have blended back into the largely Sunni civilian populations there, and are biding their time to conduct future terrorist attacks. And with few signs that the beleaguered Iraqi prime minister, Haider al-Abadi, can effectively forge an inclusive partnership with Sunnis, many senior U.S. officials warn that a military victory in the last urban stronghold of Mosul, which they hope will achieved by the end of the year, will not be sufficient to stave off a lethal insurgency.

“To defeat an insurgency, Iraq would need to move forward on its political and economic reform agenda,” Lt. Gen. Sean B. MacFarland, the top U.S. commander in Iraq, said in an email.

A return to guerrilla warfare in Iraq, while the United States and its allies still combat the Islamic State in Syria, would pose one of the first major challenges to the next U.S. president, who will take office in January. U.S. public opinion has so far supported President Barack Obama’s deployment of roughly 5,000 troops to help Iraq reclaim territory it lost to the Islamic State in 2014, but it is not clear whether political support would dissipate in a sustained effort to fight insurgents.

For U.S. diplomats and commanders, the specter of an insurgency resurrects some of the most bitter memories from the United States’ involvement in Iraq over the past 13 years. Officials voice concern about how that type of mayhem — which was led by an earlier iteration of the Islamic State and nearly crippled the Iraqi government when the United States had more than 100,000 troops in the country — could affect the stability of Iraq and the broader campaign to defeat the Islamic State, also known as ISIS or ISIL.

In a recent visit to Iraq, Defense Secretary Ash Carter acknowledged these looming challenges, noting that toppling the Islamic State in urban centers like Mosul “won’t establish control over the entirety of the territory,” and that the militants would “try to terrorize the population.”

The Islamic State is increasingly fighting less like a conventional army than “a more terrorist-type force,” Gen. Joseph L. Votel, commander of U.S. forces in the Middle East, said this week. On the battlefield, the Islamic State has redoubled its use of suicide bombers and ambushes to attack Iraqi security forces. Despite losing about half the territory it seized in Iraq, it carried out the suicide attack in Baghdad this month, one of the deadliest bombings in Iraqi history.

“When ISIS’ army is defeated in Mosul and elsewhere in Iraq, there will still be ISIS terrorist cells that will attempt to continue to carry out the kind of terrorist attacks we have seen in Baghdad and elsewhere in recent months,” Gen. David H. Petraeus, the former top U.S. commander in Iraq, said in an email.

Senior Iraqi officials agree. “Absolutely, Daesh will remain a potential threat to Iraq,” the country’s foreign minister, Ibrahim al-Jaafari, told reporters in Washington this week, using an Arabic term for the group.

U.S. military officials in Baghdad said they had not seen the Islamic State mass more than 100 troops on the battlefield since December, when a group of several hundred attacked a base in northern Iraq. “We have seen more and more of their guys with vests on trying to run into Iraqi army headquarters buildings or in the middle of a fight into a big group of soldiers,” said Col. Christopher Garver, the military spokesman in Iraq.

After losing battles to the Iraqis, some Islamic State fighters have tried to blend back into groups of civilians who have fled the violence, according to Iraqi commanders.

“We have hurt ISIS’ morale, but nobody can deny that ISIS still has its sleeper cells, and we expect anything from it,” said Lt. Gen. Abdul Wahab al-Saidi, commander of Iraqi operations in Fallujah.

“A number of ISIS fighters were found among the displaced people in Fallujah, and one of them even blew himself up,” he said. “They are criminal, and we must expect anything from the criminals because they would do anything.”

The United States and other countries in the coalition countering the Islamic State are adopting a series of measures they believe will help the Iraqis defeat the remnants of the group in the coming months.

In recent weeks, specially trained U.S. explosives experts, including a three-star Army general, and new bomb-detection devices have been sent to Baghdad to help stem suicide and car-bomb attacks. The Danes, who are part of the coalition, have begun training border patrol agents.

The first class of 300 Iraqi border patrol agents completed a four-week training course Wednesday, Garver said. The plan is to train five more similarly sized classes and use them to patrol the border with Jordan and Syria.

The U.S.-led coalition has focused intensely for months on the military campaign to retake Mosul But the dozens of defense and foreign ministers meeting in Washington this week were equally concerned with the aftermath of the fight for Mosul and the city’s security, reconstruction and governance.

Western and Iraqi officials are preparing plans to address the humanitarian needs of hundreds of thousands of Iraqi civilians displaced by the violence, and the importance of restoring local government in Mosul and other areas controlled by the Islamic State for the past two years.

“The local governance plan has to be ready to go,” said Brett McGurk, Obama’s special envoy for combating the Islamic State.

Even if the operations to take Mosul are ahead of schedule, there will almost certainly be a new U.S. president in office by the time that operation is complete. And although it is not clear how committed that administration will be to the fight in Iraq, U.S. commanders are planning for an enduring presence of forces to help the Iraqis.

“After the defeat of ISIL in Iraq, the U.S. and our partners will need to retain a presence there that can help the Iraqis secure their borders and hunt the terrorist threats within them,” MacFarland said.