Already, officials say, many Islamic State fighters who lost battles in Fallujaand 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 American officials warn that a military victory in the last urban stronghold of Mosul, which they hope will be 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 American 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 American president, who will take office in January.

American public opinion has so far supported President 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 American 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.

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