This is probably why, when asked, Mr. Turnbull could not name anyone who had asked for this reorganization. Not in the intelligence community, not from the police, not even among outside experts. A government-commissioned intelligence review also released last week made no such recommendation. Tony Abbott has unhelpfully added that he was advised against it as prime minister.

This is becoming a pattern. Hardly a month ago, the government said it would change citizenship laws to make permanent residents wait longer for citizenship, upgrade the “values” component of the citizenship test, and require the same level of English-language proficiency as university entrants. The government did this, too, by invoking national security. And again, when asked if this move was on the advice of any intelligence or law enforcement agencies, Mr. Turnbull declined to say.

What is Mr. Turnbull thinking? The answer should be obvious: politics.

Between his low standing in the polls and the insurgency he faces from the right wing of his own party (led by the man he deposed as leader, Tony Abbott, who now takes to the airwaves at least once a week to criticize his successor’s leadership), Mr. Turnbull’s position is extremely weak. He needs a prominent right-wing ally, and Peter Dutton fills this role.

There’s a more telling insight offered by the man who has lost most in this, and who was among the most vocal opponents of it within the Turnbull cabinet: Attorney General George Brandis. Pressured to support this reorganization publicly, Mr. Brandis declared the change ensures that a minister will “give 100 percent of his time and his attention to national security, both domestic, national security and border security.”

But the idea of a home affairs minister focused on national security makes sense only if we assume immigration is entirely a security problem. This points to the true ideological import of this newly formed department.

Australia began this century with a Department for Immigration and Multicultural Affairs. Back then, the department’s slogan was “Enriching Australia through Migration.” Just over a decade ago it dropped the multiculturalism portfolio entirely, creating instead a Department of Immigration and Citizenship. Now it’s to be rolled into a national security department. Thus, we can chart Australia’s public conception of migration from being a celebrated aspect of its multicultural character to a civic idea whose highest ultimate expression is citizenship to a threat to be managed.

That certainly chimes with Australia’s established rhetoric on asylum seekers, which has dominated public expression of our immigration program. And it might suit the increasingly nationalist belligerence of our age. But it does not suit Mr. Turnbull, a man who until recently was fond of celebrating Australia as “the most successful multicultural society in the world”; a man who only a few weeks ago was declaring that his party was established to be liberal, in contradistinction to conservative.

When the story of the Turnbull government is written, he will have been the prime minister who finally debased immigration in the Australian political imagination. The image last week of the prime minister draped awkwardly in military power will surely accompany that chapter. And those gas masks won’t look much like liberalism. Most likely they won’t look much like success either.