Anthony Zinni has become the latest senior military officer to leave the Trump administration, resigning his position as special envoy to help resolve disputes in the Persian Gulf and build a strategic U.S. military alliance with Arab governments in the region.

Zinni had served in numerous diplomatic roles for previous administrations and was head of the military’s Central Command, which manages U.S. forces in the Middle East, before his 2000 retirement as a four-star Marine general.

His departure follows that of two other retired Marine generals in recent weeks — Jim Mattis, who resigned as defense secretary, and John F. Kelly, who left his post as White House chief of staff last month.

President Trump once described “my generals” as key members of his administration, but none of the senior officers remain. Retired Army Lt. Gen. Michael Flynn was fired as Trump’s national security adviser less than a month into the administration. He was replaced by then-active-duty Army Lt. Gen. H.R. McMaster, who was ousted in March and replaced by former diplomat John Bolton.

Zinni described his resignation in a Monday letter to the State Department as a “formality” rather than as a protest. Efforts to resolve a dispute involving Saudi Arabia, the United Arab Emirates and other Arab countries that broke relations with Qatar in 2017 “did not make any progress,” he said in a telephone interview.

That failure undermined his other task — the building of a strategic U.S. alliance with the six nations of the Gulf Cooperation Council. Negotiations on the alliance, he said, are continuing under officials from the National Security Council and the Defense and State departments.

“I had done my thing and didn’t see anything else” to do, Zinni said.

While he received “excellent support” from Bolton and other senior administration officials, he said, “the original two guys who brought me on,” — Mattis and former secretary of state Rex Tillerson, fired by Trump early last year — “were gone.”

“Certainly I was disappointed in what happened to Jim Mattis,” he said. “He is a close friend and, I thought, the right guy for the job.”

His own departure was “nothing really dramatic,” Zinni said. “But it’s not a good time for generals, anyway.”

Deputy State Department spokesman Robert Palladino said in a statement that “General Zinni’s mission was to help introduce the concept of the Middle East Strategic Alliance,” an endeavor the statement said was “well underway.”

“General Zinni has told us he remains available should the need arise,” Palladino said. “We are grateful to General Zinni for his work with the Department of State and his service to our country.”