All 10 living former U.S. defense secretaries declared that the U.S. presidential election is over in a forceful public letter published in The Washington Post on Sunday, as President Donald Trump continues to deny his election loss to Joe Biden.The letter — signed by Dick Cheney, James Mattis, Mark Esper, Leon Panetta, Donald Rumsfeld, William Cohen, Chuck Hagel, Robert Gates, William Perry and Ashton Carter