Defense Secretary Pete Hegseth is blocking the promotions of four Army officers selected for promotion to one-star rank, a highly unusual move that appears to be targeting officers who worked for generals he does not like, current and former defense officials said.

So far this year Mr. Hegseth has removed at least 45 officers, selected by boards made up of peers, from promotion lists. More than half of those officers have been female or Black.

Mr. Hegseth’s actions appear to violate the rules governing a promotion system that is meant to be apolitical and merit-based.

Some of the Army officers pulled from the latest list appear to have worked recently for Gen. Randy George, the former Army chief of staff, whom Mr. Hegseth fired without explanation in April. The others have ties to Gen. Christopher T. Donahue, the top U.S. Army commander in Europe, who was forced to leave the Army this summer, defense officials said.

Typically, Army one-star promotion lists consist of about three dozen officers.

General George and General Donahue led the Army’s push to adapt to a future battlefield dominated by drones and artificial intelligence and the officers pulled from the recent lists played important roles in shaping their modernization vision.

An officer whose promotion was blocked commanded one of the Army’s first “transformation in contact” brigades, officials said. These 3,000-soldier units were encouraged to experiment with new drone and artificial intelligence technology, even as they trained for war.

The officer developed a new formation that use drones, electronic warfare systems and traditional scouts to see and strike the enemy deep behind the front lines. His idea was adopted by all of the Army’s infantry brigades.

The officer was also targeted, in part, because of an essay he wrote in a graduate program on the implications of political polarization on the Army’s ability to recruit and retain talent, said officials familiar with the list. It is unclear why Mr. Hegseth or members of his team found the essay objectionable.

“Military promotions are given to those who have earned them,” Sean Parnell, the Pentagon’s chief spokesman, said in a statement. “This has always been the case under Secretary Hegseth and will not change.”

In March, Mr. Hegseth struck four officers from an Army one-star promotion list that had consisted of 34 officers. Two of the officers targeted by Mr. Hegseth were Black men and two were women, prompting some senior military officials to question whether the officers were singled out because of their race or gender.

Beyond the blocking of officers for promotion to one-star rank and the firing of General George, Mr. Hegseth has fired or forced out a half-dozen senior officers — a generation of the Army’s most combat-hardened leaders — thinning the service’s top ranks for the next several years, officials said.

The personnel turmoil has also left a vacuum at the top of the Army at a moment when it is responsible for defending troops against the threat posed by cheap drones and missiles.

The Army has not had a chief of staff, the service’s top officer, since Mr. Hegseth fired General George in April, and there are no signs that he is close to filling the position, said current and former officials. Mr. Hegseth’s choice to take over as Army chief, Gen. Christopher LaNeve, does not currently have enough support in the Senate to be confirmed, officials said.

General LaNeve, who previously served as Mr. Hegseth’s senior military aide, has expressed frustration with Mr. Hegseth’s decision to block qualified officers for promotions and is concerned this will dilute the Army’s pool of experienced leaders, defense officials said.

Army Secretary Daniel P. Driscoll, the service’s top civilian leader, could leave the administration as soon as next week, officials said. Mr. Hegseth has indicated that he would like to nominate Mr. Parnell to take over for Mr. Driscoll.

Mr. Parnell served as an officer in Afghanistan, but his tenure as the Pentagon’s chief spokesman has been buffeted by controversy. Mr. Parnell recently fired three journalists at Stars and Stripes, a federally funded but independent news organization. The journalists sued the government on Thursday, claiming that the Pentagon fired them in retaliation for writing about the worsening conditions on a Navy aircraft carrier deployed to the Middle East.

Mr. Parnell also led an effort to restrict reporters’ access within the Pentagon. The moves prompted The New York Times to sue the Pentagon on the grounds that the policy violated the First Amendment.

Mr. Parnell withdrew from a Senate race in November 2021 after a judge ruled that his estranged wife should have primary custody of their three children in a case in which she accused him of spousal and child abuse. Mr. Hegseth’s interest in nominating Mr. Parnell was first reported by The Hill.

White House officials are concerned that if Mr. Driscoll leaves that Mr. Parnell will not be able to garner enough Senate votes for confirmation, leaving the Army’s top civilian and uniform military jobs open.

Many of the senior Army officers fired or forced out by Mr. Hegseth gathered Friday at Fort Bragg, N.C., for General Donahue’s retirement ceremony. The tone at the gathering was somber — one former military official in attendance described it as a “state funeral.”

In what could be one of his last acts as Army secretary, Mr. Driscoll joined the fired officers, all of them combat-tested leaders who had been groomed to lead the Army. Together they said goodbye to General Donahue.