National security adviser H.R. McMaster had been cut off at the knees, but somehow he managed to make himself visible above the lectern in the White House briefing room Tuesday.

The night before, he had declared “false” a Post report that President Trump had given highly classified information to Russia, saying “it didn’t happen.”

But Trump himself acknowledged in a tweet Tuesday morning that it did happen, saying he had “the absolute right” to share “facts” with Russia.

And so McMaster marched to the podium with a new line. Trump, he said, “shared information in a way that is wholly appropriate.” McMaster was on message: He used the phrase “wholly appropriate” nine times in his brief appearance.

But in one of these iterations he let slip why he thought Trump’s behavior was appropriate: “It is wholly appropriate for the president to share whatever information he thinks is necessary to advance the security of the American people.” In other words, it’s appropriate for Trump to say anything he wants. If Trump says it, it’s by definition appropriate.

This aligns nicely with a defense already being offered by some Trump loyalists, including Sen. James E. Risch (R-Idaho), who said Trump’s handing of secrets to the Russians was legal because “he has the ability to declassify anything at any time without any process. So it’s no longer classified the minute he utters it.”

But that dangerously distorts the president’s role. Just because it’s legal for a president to do something doesn’t mean it’s right. A president could do all kinds of things that are legal but disastrous. Our system relies on having a chief executive who practices some self-restraint that keeps him from testing the limits of power. Trump lacks such control — and that’s what is so alarming.

He can fire the FBI director, even one leading an investigation of him — though reports that he pressured James B. Comey to drop the investigation of former Trump aide Michael Flynn, if true, suggest possible obstruction of justice. He can have all kinds of undisclosed business conflicts of interest and use the presidency to enrich himself. He can share secrets with adversaries, compromising the cooperation of allies. The assumption is — or was — that a president wouldn’t.

“The troubling fact is the president has sweeping authority to wreak virtual havoc on this country and the world if he chooses to do so,” said George Washington University law professor Jonathan Turley. And Trump, he said, “seems to have a better understanding of the exemptions governing presidential conduct than the expectations.”

A president could, if he chose, legally release the names of all covert operatives spying for the United States around the world. He could start a war if he claimed an imminent threat. He could launch a nuclear attack. He could pardon and release every federal prisoner in the United States. He could recognize North Korea and cut off diplomatic relations with Germany. He could close the White House to all reporters and visitors, end all contacts with the media and declare any information — even information already published in newspapers — to be classified. He could demand the resignation of every political appointee across the government. He could, arguably, ignore the debt limit and order wiretaps on national-security grounds without court approval. And with some unspecified provocation — say, a terrorist attack — he could suspend habeas corpus.

Certainly, a president could be impeached for such actions — but that wouldn’t stop him in the short term.

Institutional restraints — the checks and balances in the Constitution — “can never fully obviate the need for certain human virtues,” Christopher Nadon, a government professor at Claremont McKenna College, wrote recently for the conservative Weekly Standard. Chief among these: self-control. The Founding Fathers, Nadon told me, wrote often of the president guided by “honor” and “decorum” and reined in by public scrutiny and humility. “They certainly thought the type of person who would get through to that office would have self-restraint,” he said.

The Constitution gives the president both specific powers (veto, appointments, treaties) and inherent powers in his role as executive. Modern presidents have stretched the bounds of executive authority. George W. Bush, guided by lawyers practicing the “unitary executive” theory, claimed new powers to conduct military trials and even to torture detainees. Barack Obama tested the bounds with executive actions on health care and immigration.

But those abuses both seem quaint now that we have a president governing with a terrifying combination of impulsivity and ignorance. McMaster, in the closing lines of his briefing Tuesday, offered an admission about the intelligence Trump gave Russia: “I should just make, maybe, the statement here that the president wasn’t even aware, you know, where this information came from.”

So Trump gave secrets to Russia — but that’s okay because he didn’t know what he was doing. This is supposed to make us feel better?

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