In times of crisis, the leaders of the military and intelligence communities try to put aside their differences, often many and sundry, and work together for the good of the country. That’s what’s happening today with a remarkable group of retired generals, admirals and spymasters who have signed up for the resistance, telling the president of the United States, in so many words, that he is not a king.

Thirteen former leaders of the Pentagon, the C.I.A. and the F.B.I. have signed an open letter standing foursquare against President Trump, in favor of freedom of speech and, crucially, for the administration of justice. They have served presidents going back to Richard M. Nixon mostly without publicly criticizing the political conduct of a sitting commander in chief — until now.

“We have never before seen the approval or removal of security clearances used as a political tool, as was done in this case.”

They rebuked Mr. Trump for revoking the security clearance of John Brennan, the C.I.A. director under President Obama, in retaliation for his scalding condemnations and, ominously, for his role in “the rigged witch hunt” — the investigation into Russia’s attempt to fix the 2016 election, now in the hands of Robert S. Mueller III, the special counsel. The president’s latest attempt to punish or silence everyone connected with the case, along with his fiercest critics in political life, will not be his last.