Revoking security clearances shows president's autocratic tendencies.

The autocratic tendencies of this country's chief executive grow stronger by the day, this time taking the form of revoking security clearances as punishment for those whose words embarrass, displease or reveal.

But in revoking the clearance of former CIA Director John Brennan, President Donald Trump has crossed a line among those trained to spot governments in crisis.

At least 15 of the nation's top intelligence officials — a type not known for high drama — have banded together in a letter of protest against Trump's decision on Brennan, sending a distress signal this nation can ill afford to ignore. These are all individuals who have served their country, deemed worthy of its highest security clearances, all signaling now that something is terribly wrong.

The growing list itself is stunning. It includes six former CIA directors — two of them generals. Five are former deputy directors. Former director of National Intelligence James Clapper has signed on, as has Robert Gates, who is a former CIA director and former defense secretary. They note in the letter that whether they have been outspoken about perceived national security threats or remained quiet, all agree that "the president's action has nothing to do with who should or should not hold security clearances — and everything to do with an attempt to stifle free speech."

There it is again, free speech. Trump simply cannot seem to stomach the fact that in this country, even as president, he cannot silence his critics. He cannot force them — not through signed agreements or loyalty tests or under threat of punitive actions — to stop speaking their truth. There had been no indication that Brennan has abused his security clearance or revealed classified information.

"We have never before seen the approval or removal of security clearances used as a political tool, as was done in this case," the letter stated. Such is the consensus of those who have led U.S. intelligence efforts stretching back to CIA Director William Webster, who directed the CIA under Presidents Ronald Reagan and George H.W. Bush, led the FBI before that and has led the Homeland Security Advisory Council since 2005.

Individuals such as these retain their clearances both as a professional courtesy and because, collectively, they are a living bank of knowledge, experience, strategy and tested risk assessment that can be tapped whenever the need arises. Brennan alone had more than two decades with the agency. It is foolish for Trump to reject all that over perceived insults to his ego.

There are others he has threatened, such critics as former National Security Agency chief Michael Hayden and former deputy attorney general Sally Yates, who was dismissed for insubordination after ringing the alarm (rightly, as it turned out) on Trump's original travel ban. And yet, a Trump pet, former National Security Advisor Michael Flynn, who committed a federal felony by lying to the FBI, retains his clearance.

There is another, darker possibility raised by the president himself: that he is going after those whom he blames for what he calls the "rigged witch hunt," otherwise known as the federal investigation into Russian election interference.

No matter the reason, the U.S. remains a nation of laws. It does not govern by presidential whim or tantrum. In an even more extraordinary action, Ret. Navy Admiral William McRaven, former commander of U.S. Joint Special Operations Command, who oversaw the Navy SEAL raid that took out Osama bin Laden, said last week that Brennan was a man of "unparalleled integrity." He added that he would "consider it an honor if you would revoke my security clearance as well, so I can add my name to the list of men and women who have spoken up against your presidency … You have embarrassed us … humiliated us on the world stage and worst of all, divided us as a nation."

Statements such as these are not made lightly by career military officials about a commander in chief. It is inconceivable that so many distinguished leaders who served under so many different presidents and from different parties, would take such action out of pique. These are not men who stand in fear of the next election cycle, or who are beholden to special interests or foreign governments. Instead, they have risked their lives for their country and they are speaking out.

It is beyond time that the American public — and Congress — listen.