WASHINGTON — The Pentagon policy official overseeing the military’s efforts to combat the Islamic State was fired Monday after a White House appointee told him the United States had won that war and that his office had been disbanded, according to three people briefed on the matter.

The ouster of Christopher P. Maier, the head of the Pentagon’s Defeat ISIS Task Force since March 2017, came just three weeks after President Donald Trump fired Defense Secretary Mark T. Esper and three other Pentagon officials, and replaced them with loyalists.

In a statement late Monday, the Pentagon said that Acting Defense Secretary Christopher C. Miller had accepted Maier’s resignation and that his duties would be folded into two other offices that deal with special operations and regional policies. Those offices are led by Ezra Cohen-Watnick and Anthony J. Tata, two of the Trump appointees who have been promoted in the recent purge.

The Pentagon statement said the transition reflected the success of the U.S.-led effort to crush the terrorist state that ISIS created in large swaths of Iraq and Syria.

But Maier’s supporters say he was summarily forced out of an important but low-profile job that required navigating the shoals of Washington’s counterterrorism bureaucracy as well as flying off to combat zones, including northeast Syria and Iraq to work with precarious partners on the ground in the fight against the Islamic State group.

“Chris is a nonpartisan professional and carries years of institutional knowledge on an exceedingly complex set of issues,” Brett H. McGurk, Trump’s former special envoy to the coalition to defeat the Islamic State, said in an email. “It really makes no sense to force out someone like that 50 days before a transition to a new administration.”

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Maier’s task force was responsible for overseeing policy and strategy development as well as international negotiations regarding the fight against ISIS. At least one member of President-elect Joe Biden’s transition team expressed concerns privately Monday night over Maier’s firing.

Maier’s team served as a clearinghouse for the government’s counterterrorism operations and policies. It was in the midst of answering dozens of questions from the incoming Biden team about the status of terrorist threats, relations with allies and counterterrorism missions when his team was disbanded. Now team members will be scattered across the Pentagon bureaucracy or returned to their home agencies.

Whether deliberate or not, the move by the newly promoted Pentagon leadership to eliminate that central hub will almost certainly slow the flow of counterterrorism information to Biden transition aides in the coming weeks, several officials said.

The Pentagon statement said Miller thanked Maier for his service. But a senior U.S. official said relations were strained between Maier and Miller, a former Army Green Beret who later served in counterterrorism jobs at the Pentagon and the White House’s National Security Council before Trump picked him to replace Esper in the Pentagon’s top civilian job.

Maier, 44, an Air National Guard intelligence officer who has worked in counterterrorism jobs in Republican and Democratic administrations for two decades, declined to comment. He recently completed a 90-day reservist tour in Kuwait. His firing was reported earlier by CNN.

While Maier’s backers expressed dismay at the manner of his dismissal — summoned midday Monday by the White House liaison officer at the Pentagon, Joshua Whitehouse, and told to clear out that day — they also criticized the new Defense Department leadership for playing to Trump’s overly optimistic assessment of the Islamic State’s status. The officials spoke on condition of anonymity because they were not authorized to publicly discuss the situation.

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When Miller announced last month that the United States would draw down to 2,500 troops in both Iraq and Afghanistan by Jan. 15, he trumpeted the demise of the Islamic State.

“Thanks to our more than 80 partners in the Defeat-ISIS Coalition, we have destroyed the ISIS caliphate and will ensure they never again gain a foothold to attack our people,” Miller said in remarks Nov. 17.

But two months earlier, in late September, while still in his previous job as director of the National Counterterrorism Center, Miller struck a more sobering note in testimony to a House committee: “ISIS has repeatedly demonstrated the ability to rebound from severe losses over the past six years by relying on a dedicated cadre of veteran midlevel commanders, extensive clandestine networks, and downturns in CT pressure to persevere,” referring to counterterrorism pressure.

Other counterterrorism officials estimate that the Islamic State, despite having lost its territorial control in Iraq and Syria, still has as many as 10,000 guerrilla fighters there and maintains resilient affiliates across East and West Africa and Afghanistan.

“Chris is a straight-up pro,” Nicholas J. Rasmussen, a former director of the National Counterterrorism Center, said of Maier. “The idea this is justified because we’ve won the war against ISIS strains credulity.”

Pentagon officials insist they are not taking ISIS lightly, but that the moment was right to reorganize the department’s counterterrorism policy efforts. The disbanding of Maier’s office and his 15 or so staff members follows a similar consolidation at the State Department, where the counter-ISIS duties carried out by James Jeffrey were transferred to the department’s permanent counterterrorism coordinator when Jeffrey retired last month.

“The Department of Defense will continue to engage with our partners and allies to ensure the lasting defeat of ISIS and encourage the repatriation of foreign terrorist fighters for prosecution,” said the Pentagon statement.