The “special relationship” between the U.S. and the United Kingdom is not just an expression. It’s the foundation of Western defense, diplomatic, economic, trade and cultural cohesion that’s more important than ever in an era of increasing geopolitical instability.

The relationship is a bond between nations and peoples, and it belongs to no party. It has been bipartisan on both sides of the pond, enduring regardless of whether a Republican or a Democrat resided in the White House and whether a Conservative or a Labor prime minister occupied No. 10 Downing Street.

So it’s serious but sadly unsurprising that presumptive GOP presidential nominee Donald Trump told Britain’s ITV television that he expects to have a poor relationship with British Prime Minister David Cameron.

The candidate was responding to Cameron’s comments of a few months ago that Trump’s proposed temporary ban on Muslims entering the U.S. was “divisive, stupid and wrong.”

Cameron is right, though he might have chosen his words more artfully. Trump’s proposed policy, essentially a religious test, not only tears at the fundamental fabric of American society, but also would likely further inflame anti-American sentiment in the Muslim world — including with allies — and also in Europe, especially in London, which just last week elected a Muslim mayor, Sadiq Khan.

Cameron is not the only key European leader with whom Trump might have problems. In a Twitter tantrum last December after he was not named Time magazine’s “Person of the Year,” Trump tweeted of German Chancellor Angela Merkel: “I told you @TIME Magazine would never pick me as person of the year despite being the big favorite They picked person who is ruining Germany.”

Should he be elected, Trump would quickly learn just how much he — and America — needs the U.S.-U.K. special relationship and how essential it is to further tighten transatlantic bonds.

Principled conservatives, who can rightly take pride in Republicans’ role in building and maintaining these ties in the postwar era, should reflect carefully about whether they dare trust them to such an undisciplined, unwise brawler.