A retired Marine four-star general who once headed U.S. Central Command and later served as an envoy to work the Israel-Palestinian conflict has resigned from his current post working on a Qatar dispute.

Retired Gen. Anthony Zinni told CBS News that he realized he could not help to resolve the dispute, “because of the unwillingness of the regional leaders to agree to a viable mediation effort that we offered to conduct or assist in implementing.”

The dispute revolves around accusations that the Qatar government has supported militant groups tied to terrorist attacks in the region.

In June 2017 Egypt, Saudi Arabia, Bahrain and the United Arab Emirates cut ties with Qatar.

Former Defense Secretary Jim Mattis and then-Secretary of State Rex Tillerson first requested Zinni to serve as a special adviser to the secretary of state on Middle East concerns in 2017.

Both Tillerson and Mattis have since resigned.

During his tenure, President Donald Trump also looked to the former general to help build an ‘Arab NATO’ to counter Iranian influence in the Middle East.

The concept focuses on creating a security and political alliance between the Egypt, Jordan and the Gulf Arab states. That would join mostly Sunni Muslim majority states against the Shia-majority Iran.

Trump’s administration dubbed the fledgling group the Middle East Strategic Alliance.

Retired Marine Gen. Anthony Zinni, right, then-commander-in-chief, Central Command, testifies on Capitol Hill, Tuesday, March 3, 1998, before the Senate Armed Service Committee hearing on military strategy and operational requirements. (Tyler Mallory/AP)
Retired Marine Gen. Anthony Zinni, right, then-commander-in-chief, Central Command, testifies on Capitol Hill, Tuesday, March 3, 1998, before the Senate Armed Service Committee hearing on military strategy and operational requirements. (Tyler Mallory/AP)

Zinni told CBS News that he was not needed to continue work on the MESA, as other administration officials would continue that project.

The retired general told Task & Purpose that he was “disappointed” about Mattis’ recent resignation, calling him the “right guy for the job.”

But he said Mattis’ departure didn’t affect his own resignation.

Zinni, 75, is a Vietnam War veteran who later served as the commander in chief of U.S. Central Command.