Retired Marine General 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, a Vietnam veteran, 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 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.

“The original two guys who brought me on, are gone,” said Zinni. “Certainly I was disappointed in what happened to Jim Mattis. 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.”

A former commander of U.S. Central Command, Zinni had previously served as a special envoy during the Bush administration to work on Israeli-Palestinian peace negotiations.

Zinni joins a growing list of national security personnel that have left the Trump administration over the past month, including Mattis and Brett McGurk, the former special presidential envoy for the Global Coalition to Counter ISIS.

The Department of Defense’s chief of staff Rear Adm. Kevin Sweeney also resigned last week, in addition to Pentagon spokeswoman Dana White, who announced her resignation at the end of December.

Former Army Gen. Stanley McChrystal, who commanded U.S. forces in Afghanistan from 2009 to 2010, called Trump “immoral” and unfit to lead, to which Trump responded that McChrystal had a “big, dumb mouth.”