Retired admiral: It was a 'sad moment' when Gen. Kelly sold out his principles to be Trump's attack dog
White House chief of staff John Kelly gives a press briefing. Image via screengrab.

Former Joint Chiefs of Staff and Ret. Admiral Michael Mullen told ABC News on Sunday that he has strong reservations about military generals being called on to serve in civilian administrations like President Donald Trump's.

Mullen touted the military's apolitical stance on national issues and said that White House Chief of Staff -- and retired Marine Corps general -- John Kelly, National Security Adviser and U.S. Army General H.R. McMaster and Secretary of Defense Jim Mattis -- a retired Marine Corps general -- are all "great Americans" for wanting to serve their country, but warned that they are at risk of becoming political pawns.

He called out Kelly for his defense of Trump when the president was at odds with the family of slain U.S. Army Ranger Sgt. La David Johnson.

Watching that was "a really sad moment for me," Mullen said. Kelly made it clear that nothing was off-limits in the interest of defending Trump "no matter what."

"That doesn't mean generals and admirals can't serve," in the White House, Mullen said. "They certainly have in the past. But it's particularly difficult right now because of the politics of the town. And there's nothing that seemingly is not able to be politicized in the current environment."

Watch the video, embedded below: