President Trump, flanked by Defense Secretary Jim Mattis, speaks during a Cabinet meeting at the White House on Dec. 6, 2017. (Jabin Botsford/The Washington Post)

Defense Secretary Jim Mattis has resigned. His letter made clear he disagrees in a fundamental way with President Trump over basic aspects of national security policy, including how to treat friends and enemies.

Officials in the Trump administration have departed at an unusually high rate. But Mattis’s resignation is different. Here’s why.

1. Resignations in protest are rare.

In previous administrations, there has been no shortage of times we might have expected a resignation in protest over decisions made by the White House, but these resignations did not materialize. For example, Secretary of State Colin Powell might have resigned in the run-up to the Iraq War, but he did not. In the Carter administration, Secretary of State Cyrus Vance resigned over the Iranian hostage rescue mission — an exception that proves the rule.

2. Presidents work hard to avoid resignations in protest.

The rarity of these dramatic resignations is no accident. As my research shows, presidents know that advisers’ speaking out against their policies can have damaging effects, particularly when criticism comes from a high-profile adviser who commands respect and attention. Such criticism, or in the extreme, a resignation in protest, serves as a kind of “fire alarm” that tells people who don’t pay much attention to foreign policy that they should sit up and take notice.

So presidents try to keep that fire alarm from going off in the first place. They give advisers some of what they want — as when George W. Bush tried to keep Powell on board by going to the United Nations in the run-up to the 2003 Iraq War, and ended up trying to use Powell’s skepticism to his advantage by making him the public face of the administration’s case. Or when Barack Obama made sure to keep David Petraeus and Robert Gates on board with his Afghanistan “surge” decision.

3. What’s unusual is the bargaining between the president and his own team has broken down so publicly.

The Mattis resignation is unusual precisely because these types of dramatic fire alarm moments are not supposed to happen if presidents successfully bargain with their advisers.

But this is by no means the first fire alarm on national security we’ve seen in the Trump era. Consider what has happened just this fall. In September, the anonymous editorial in the New York Times revealed concerns over Trump’s “ill-informed and occasionally reckless decisions.”

In November, Trump sent Mattis and Secretary of State Mike Pompeo to Congress to reiterate his belief that there was no “smoking gun” evidence that Saudi Crown Prince Mohammed bin Salman, was behind the killing of journalist Jamal Khashoggi. Senators didn’t buy it, and that led to calls for CIA Director Gina Haspel to come brief senators.

Following Haspel’s testimony, which appeared to contradict Trump, several prominent Republican senators who focus on national security made harshly critical comments about the Trump Saudi policy — and followed it up with a symbolic but important vote rebuking the administration on U.S. support for the Saudi-led war in Yemen. As Sarah Binder wrote here at the Monkey Cage, “in our era of heightened partisanship and polarization, any dissent within the president’s own party about critical war powers is unusual, even startling.”

Taking friendly fire from Republican senators or his own advisers is not supposed to happen — and, as I wrote in the case of the Haspel testimony, certainly not when it was avoidable.

But Trump seems unable to keep advisers in the fold through bargaining or persuasion. In the case of Mattis, it appears he stopped trying, making the decision to withdraw from Syria over Mattis’s objections and without the kind of interagency process and orderly planning that would at least make such a decision more palatable to the Pentagon. Trump also passed over Mattis’s choice for the next chairman of the Joint Chiefs of Staff — a move that might have softened other blows.

Of course, withdrawing from Syria is a long-held Trump policy preference, and is certainly the president’s prerogative as commander in chief.

But as a political matter, making policy changes without bringing advisers or other key allies on board raises the risk of fire alarms going off. Some of these fire alarms are still mostly audible to those inside the Beltway — as in the Washington parlor game of guessing who wrote the Times op-ed.

With the Mattis resignation, these alarms — already getting louder and more frequent — are approaching full klaxon levels. This makes it increasingly difficult for Congress and the American public to ignore them.

Read more:

What Mattis’s resignation tells us about how Trump is damaging the military’s credibility

Three reasons you should be startled by how the Senate rebuked Trump

Last week showed how Trump’s management of his team is hurting his own foreign policy. It’s only going to get worse.

5 reasons to be concerned about deploying U.S. troops along the southern border

Why is the Senate challenging Trump on Yemen? Here’s what you need to know.

Sure, Trump’s advisers aren’t the first to push back against a president. But what’s happening now is completely unprecedented.

Eight questions about Donald Trump’s decision-making in a foreign policy crisis