Fifty former national security officials, all of whom have served under Republican presidents from Richard Nixon to George W. Bush, have signed a letter calling Donald J. Trump unqualified to be president and warning that, if elected, “he would be the most reckless President in American history.” The letter offers a withering critique of the GOP nominee, saying he “lacks the character, values and experience” to be president and declaring their conviction he would be dangerous “and would put at risk our country’s national security and well-being.” The signatories state flatly that none intend to vote for Trump in November. But they say they understand the frustrations of many Americans.