At a Coast Guard event in May, where President Donald Trump was presented with a ceremonial sword, John Kelly — at the event in his role as secretary of Homeland Security — memorably quipped, “Use that on the press, sir.”

It was a hot-mic moment in line with the “tough-talking general” persona that’s endeared Kelly to Trump — so much so that the retired Marine general is now White House chief of staff.

After six months of an administration marked by intra-White House intrigue and infighting, Trump announced late Friday in a tweet that Kelly would replace Reince Priebus to lead Trump’s White House staff.

Kelly’s elevation is a reminder that the core of Trump’s political identity remains his tough stance on immigration: As secretary of DHS, Kelly became a reliably blunt spokesperson for the Trump administration’s expansion of immigration enforcement. But arguably more than that, it’s a reminder of what President Trump thinks strength looks like.

Kelly has a tendency, at least in public, to treat criticism as a personal affront to the people working to keep America safe. In an administration that (while beleaguered by leaks) is ever more convinced that both it and America are under siege, that outlook might not be what the White House actually needs — but it makes sense that it’s the outlook Trump thinks the White House needs.

Kelly isn’t used to domestic policy — or the open debate that comes with it

Kelly was one of a triumvirate of generals named by Trump to key Cabinet-level positions before his inauguration: Gen. James Mattis was named secretary of defense, and Gen. Mike Flynn was named national security adviser (a position he was fired from in February, and replaced with Army officer H.R. McMaster).

But while Mattis and Flynn had experience working in defense and intelligence, Kelly didn’t have experience with domestic national security policy. He’d worked extensively in Central America as head of US Southern Command, but he knew little about domestic immigration policy — not to mention the grab-bag of other things with which DHS is tasked.

More importantly, he didn’t have experience with the open debate and criticism that domestic policy engenders. While the government often gets some deference from the press, Congress, and the public on national-security issues, that deference isn’t absolute — especially 16 years after 9/11. And that criticism sometimes extends to the people carrying out the policy.

The saying “Politics stops at the water’s edge” still describes a powerful norm: Soldiers aren’t criticized for carrying out the policies of their governments, and it takes egregious abuses of rules of engagement for individual soldiers to be shamed. But that doesn’t apply to people within the US — even law enforcement officers.

In his six months at DHS, Kelly never really accepted that norm. He presented himself as a “soldiers’ general”: He said that the morale of DHS employees (including, crucially, the immigration agents who had spent the Obama administration clamoring to deport more unauthorized immigrants) was his top priority, and he accepted unquestioningly the perspectives of people on the ground as the most important and authentic.

While Trump has credited Kelly with successfully implementing an immigration policy turnaround at DHS, Kelly and department leadership really haven’t done much themselves. They’ve simply granted greater latitude to officers in the field — and defended them vociferously whenever criticized.

There’s been plenty of criticism. Kelly — who wasn’t even briefed on the first iteration of Trump’s “travel ban” executive order until Trump was in the midst of signing it — came under fire for the aggressive detention and interrogation of people from majority-Muslim countries by Customs and Border Protection agents during the week the ban was in effect. He’s been criticized by Democratic members of Congress for ICE’s actions to apprehend immigrants in courthouses and near churches, and for their occasional attempts to arrest and deport immigrants protected under Obama’s Deferred Action for Childhood Arrivals program.

But with each round of criticism, Kelly’s response has gotten more forceful. In the past few months, he’s overtaken Sessions (who’s fallen out of favor with the mercurial president) as the face of the administration’s immigration policy. And he presents it with a blunt, law-and-order morality and an edge of outrage toward anyone who might dare question those who keep America safe.

He likes to say that he doesn’t deport immigrants, the law does. In a May speech, he blamed the department’s morale problem on the public trusting “criminals” over agents: “When you discourage, when you disable, when you unjustly criticize and default to believing the initial reports as opposed to defaulting to believing the stories told by my professionals, what else do you expect?”

Kelly’s rhetoric has often glossed over or contradicted the policies his department carried out, or he’s simply refused to believe accounts of officer abuses. But his simple us-versus-them tone isn’t dissimilar to Trump’s own, or to the other figure gaining more influence over the president right now: communications director Anthony Scaramucci, who called New Yorker reporter Ryan Lizza Thursday to demand as an “American” that Lizza tell him who’d leaked gossip about a dinner.

It’s not clear that Kelly or anyone can actually discipline Scaramucci or the other people vying for influence with the president. It’s not clear the president wants him to — he totally rejected Priebus’s attempts to control access to the Oval Office. But as an exponent of the way Trump sees the world, and as someone who can represent the administration on television, it’s not hard to see why Trump thinks now is the time for a general who, at his worst, reacts to criticism as a national security threat.