Selva, who served as vice chairman of the Joint Chiefs of Staff until July 2019, appears on a list of 489 national security experts - including former military leaders, ambassadors and White House officials - who signed a letter being released on Thursday that declares Trump "not equal to the enormous responsibilities of his office."

"Thanks to his disdainful attitude and his failures, our allies no longer trust or respect us, and our enemies no longer fear us," reads the letter by the group, called National Security Leaders For Biden.

Selva could not be reached for comment.

Other groups of former national security leaders have endorsed Biden and criticized Trump, but it is remarkable that a recently retired four-star general like Selva - who was the Pentagon's No. 2 military officer - would publicly endorse any candidate and sign onto a letter condemning a president he served.

Beyond Selva, retired Admiral Paul Zukunft, who served as commandant of the Coast Guard from 2014 until 2018, also appears on the long list of signatories.

"I'm not a Republican and I'm not a Democrat," Zukunft told Reuters in explaining his decision.

"It really comes down to the fundamental values of our nation and our concern that those values are being compromised."

Like Zukunft, Selva was not initially selected for his position by Trump, who inherited military leaders picked by his Democratic predecessor, Barack Obama. Trump renewed Selva for a second, two-year term in 2017.

Other prominent retired military officers endorsing Biden in the letter include: Samuel Locklear, a retired Navy admiral who led the US Pacific Command until 2015, and Peter Chiarelli, who retired in 2012 after serving as the Army's vice chief of staff.

Strained ties with pentagon

Although Trump describes himself as a champion of the US military and boasts of hefty military spending, his relationship with the Pentagon has been strained. He has ridiculed top generals, ignored their advice on some key issues, and questioned their commitment to their soldiers.

Earlier this month, Trump sought to underscore his support among rank-and-file soldiers.

"I'm not saying the military's in love with me. The soldiers are," Trump told reporters. "The top people in the Pentagon probably aren’t because they want to do nothing but fight wars so all of those wonderful companies that make the bombs and make the planes and make everything else stay happy."

Asked about Selva's endorsement of Biden, White House spokeswoman Kayleigh McEnany said she could not speak to one individual and defended Trump's support for US troops.

"The rank and file of this military love this president," she told reporters at the White House.

Biden's campaign has been trying to build a broad coalition of liberals, moderate Republicans and independents. When he accepted the Democratic nomination at his party's convention in August, Biden said that, if elected, he would be a president for all Americans, not just for the Democratic base.

Susan Rice, former US national security adviser under the Obama administration, told Reuters many more prominent former officials wanted to join the list but missed a deadline ahead of the letter's public release.

She attributed the strong turnout to deep concerns among former US military, diplomatic, and intelligence officials.

"On the military side, there were many who felt strongly that this was a moment in our history when our democracy and our Constitution was on the line," said Rice, co-chair of National Security Leaders for Biden. 

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