WASHINGTON — So how exactly does this work, Donald Trump’s plan to keep America safe from Islamic terrorism by barring entry to all Muslims? He explained it Tuesday on TV. The immigration official will ask the foreigner if he’s a Muslim.

“And if they said, ‘yes,’ they would not be allowed in the country?”

Trump: “That’s correct.”

Brilliant. And very economical. That is, if you think that bloodthirsty terrorists — “people that believe only in Jihad, and have no sense of reason or respect for human life,” as Trump describes them — will feel honor-bound to tell the truth to an infidel immigration officer. They kill wantonly but, like George Washington, cannot tell a lie. On this logic hinges the great Maginot Line with which Trump will protect America from jihad.

I decline to join the chorus denouncing the Trump proposal as offensive and un-American. That’s too obvious. What I can’t get over is its sheer absurdity.

Dozens of others have already pointed out how strategically idiotic is Trump’s exclusion principle. Absent a renewed Christian crusade against radical Islam, the war on terror will only be won in alliance with moderate Muslims. Declaring them anathema is not the best beginning to coalition-building.

To treat “no Muslims allowed” as a serious idea is to give credit to what is little more than a clever stunt by a man who saw Ted Cruz beating him for the first time in the Iowa Monmouth poll and five hours later decided it was time to seize the stage again.

This got the thinkers going again. National Review’s Andrew McCarthy, whom I (otherwise) hold in considerable esteem, spent 1,000 words trying to tart up the ban in constitutional and statutory livery, stressing — hilariously — that he is dealing with the Trump proposal “in its final form.” As if Trump’s barstool eruptions are painstakingly vetted, and as if anything Trump says about anything is ever final.

Take his Syria policy. In September, he said we should wash our hands and just let Russia fight the Islamic State. Having, I assume, been subsequently informed that Vladimir Putin’s principal interest — and target — is not the Islamic State but the anti-Assad rebels, Trump now promises to “bomb the s— ” out of the Islamic State.

Whatever the final outcome, Trump’s campaign has already succeeded, indelibly affecting both this race and the Republican future. At a time of economic malaise at home and strategic collapse abroad, Trump has managed to steer the entire GOP campaign into absurdities, like mass deportation of 11 million illegal immigrants, and impossibilities, like the exclusion of Muslims from our shores.

“No Muslims allowed” is the perfect example. President Obama’s Oval Office address on Sunday night marked a new low in his presidency. The shopworn arguments, the detached tone, the willful denial that there might be anything wrong with his policy was deeply unsettling for left, right and center. Even The New York Times had to admit “Obama’s Plans to Stop ISIS Leave Many Democrats Wanting More,” which is Timesese for Democrats Stunned by Vacancy in the Oval Office. Here was an opportunity for the Republican field to launch an all-out takedown of the Obama (and Hillary Clinton) foreign policy.

Within less than a day that opportunity was wiped out. Once again, it’s the Donald Show.

Charles Krauthammer is a columnist for The Washington Post. His email address is: letters@charleskrauthammer.com.