It is rare for recently-retired high-ranking military officers to publicly endorse a political candidate or criticize a commander in chief they served. While former top military leaders have endorsed Biden and blasted Trump, Selva is the highest-ranking Trump-era leader to do so. Trump's predecessor, Barack Obama, selected both Selva and Zukunft for the jobs, but they served well into the Trump administration.

"We love our country. Unfortunately, we also fear for it,” the open letter said. "The COVID-19 pandemic has proven America needs principled, wise, and responsible leadership. America needs a president who understands, as President Harry S. Truman said, that ‘the buck stops here.’”

Other signatories included two Obama defense secretaries, Ash Carter and Chuck Hagel, as well as Eric Edelman, who served as undersecretary of defense for policy in the George W. Bush administration.

The signatories called Biden a wise and honest leader who believes in personal responsibility and "always put the nation's needs before his own." They contrasted Biden with Trump, who they said has demonstrated he is "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," they wrote, citing failures on climate change and North Korea's nuclear program. They also slammed Trump for ceding influence to Russia and starting a trade war with China that has hurt American workers.

By contrast, Biden "would never sell out our allies to placate despots or because he dislikes an allied leader," they wrote.