Editors’ note: This post has been updated to reflect the news that Secretary Esper reversed a decision to send troops in Washington, DC, back to their bases; and that former Secretary of Defense James Mattis issued a statement strongly criticizing President Trump.

At a Pentagon news conference Wednesday morning, Secretary of Defense Mark T. Esper said he opposed invoking the Insurrection Act and using active-duty military forces to help calm the largely peaceful protests that have been taking place around the country. Esper’s comments directly contradict President Trump, who in a nationally televised speech Monday threatened to use the military to “quickly solve the problem,” implicitly suggesting that he would invoke the 1807 law.

Esper’s comments also came after many criticized him for walking across Lafayette Square with the president and posing for a photo in front of St. John’s Episcopal Church, as well as using language like “we need to dominate the battlespace” on a Monday call with governors. On Tuesday evening, James Miller, a member of the Defense Science Board, which advises the Pentagon, wrote to Esper a letter, published in The Washington Post, to resign his position and to urge Esper to “consider closely both your future actions and your future words.”

It is tempting to dismiss Esper’s comments as words rather than action. He is not resigning in protest, as his recent predecessor, Jim Mattis, did in December 2018.

However, for Esper to give televised remarks from the Pentagon podium — something that is rare in this administration in normal times — is a significant development. Here’s why:

1. Resignation in protest is very, very rare for high officials.

Mattis’s resignation was quite unusual. In the United States, high officials rarely resign in protest. As I wrote when Mattis resigned, we often expect resignations in protest that do not materialize, as in the case of Colin Powell in the Iraq War.

Miller’s resignation is the exception that proves the rule — he was not an official directly involved in the events that he cited in his letter.

There are many self-interested reasons officials do not resign, including protecting their careers, livelihoods and loyalties. After all, presidents are likely to select advisers who are inclined to support them. Presidents also work hard to avoid resignations in protest, sometimes bargaining with their advisers over policy and making sure the advisers are just happy enough to stay onboard.

The costs of resigning in protest are high, but the benefits are particularly low in a highly polarized environment, where the protest part of a resignation in protest may not change many minds. Furthermore, news overload and news fatigue in the Trump era makes it hard for what would normally be big stories to break through. Why should an official resign in protest as a message if the audience isn’t likely to hear what she is saying?

Of course, there are other reasons resigning in protest might be considered appropriate, and Esper may yet do so. But such events are uncommon for a reason.

2. Public, on-camera opposition is also rare — and this will get attention.

When Cabinet officials oppose the president or even resign, they more often do so in print — often anonymously. (Remember that anonymous op-ed writer?) Even Mattis issued his disagreement via a letter of resignation. Furthermore, Mattis disagreed with Trump over the president’s decision to remove troops from Syria and Trump’s antagonism toward U.S. allies.

But most voters do not pay much attention to foreign policy. Esper’s comments were about using U.S. troops to suppress protests — an issue far more Americans know and care about.

3. Advisers can affect public and congressional opinion.

When advisers speak out in public, their words have consequences. In my research on the politics of the use of force, I have found that advisers are significant political actors whose statements can affect public opinion. Jim Golby, Peter Feaver and Kyle Dropp have shown that public cues from the military shape public views of using force — especially when the military opposes using force.

This might not tell us about the effects of Esper’s words. He is a civilian, and all this research examines using military force abroad rather than at home.

That said, it is plausible that Americans are more likely to pay attention when Esper speaks out on camera, contradicting the president’s stated preference to use the military on domestic soil to quell protests, than when an adviser talks about the use of force abroad (which we already know can affect public attitudes). Esper’s news conference will probably get attention among members of Congress, as well.

Late yesterday, former Secretary of Defense James Mattis issued a statement strongly criticizing Trump, adding even more weight to the series of military and civilian criticism of the president’s approach to the protests coming from current and former Pentagon officials. While Mattis’ comments are also very significant, it is still notable that Esper spoke while still in office, and on camera.

4. Esper’s news conference shows how Trump (mis)manages his advisory team.

All presidents face internal disagreement — in fact, some presidents welcome it. However, when disagreement happens, presidents have the task of managing the internal political fallout. They want to project unity, to help generate support for their policies. As my research on presidential bargaining with elites shows, sometimes presidents have to make concessions on policy, or how policies are implemented, to get that unity. This is one reason advisers have power and most often stay in their posts.

Trump does not play this bargaining game. Instead, he generates unity by demanding loyalty and firing those who disagree with him (as may yet happen with Esper). As Jonathan Bernstein has argued, this is one of the ways Trump fails to exercise his presidential power to his own maximum advantage.

By lunchtime, there were already news reports that Trump was, predictably, very unhappy with Esper’s statements. Esper later went to the White House and then reversed an earlier decision to send troops back to their bases.

Still, there were signs that the message had some effect. Some Republican senators went on the record to say that Trump should not fire Esper, and that they agreed with Esper’s view of how to handle the protests. All these statements raise the political costs to Trump of both firing Esper and taking a harsh approach to the protests.

Up to this point, Republicans in Congress have mostly been willing to line up to support Trump, in part because they fear the electoral consequences if they don’t. But as Michael Tesler showed here at TMC and Dan Drezner has also argued, Trump’s handling of the protests is likely to hurt rather than help him politically.

It’s usually a good bet that Republicans will continue to back Trump. But a public breach with the secretary of defense in the middle of a major national crisis involving the use of military force on American soil may still be a politically dangerous moment for the president.

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