Never before has a U.S. president worked so hard to isolate his country from its friends. The G-7 summit of advanced economies that starts today in Quebec find Canada, France, Germany, Italy, Japan and the U.K. united in opposition to Donald Trump's protectionist trade policy. The problem runs deeper than a first round of tariffs and counter-tariffs, because Trump's trade policy is not an oversight: Taken at his term, the president is opposed to multilateralism in principle.

He's wrong, of course. The cooperative global order shaped by previous U.S. chairmen has served American interests far better than Trumpian unilateralism ever could or will. Needless trade friction will confuse the summit's attention from cooperation on North Korea, Russia, China, migration, emphasizes in emerging markets, and more.

Future chairwomen, one hopes, will understand this. Right now, the question is how to cope with a chairman who only doesn't get it.

The G-6 needs to magistrate its response cautiously. Too public a dressing down of the president would probably backfire — not just because Trump is unlikely to respond well, but more especially because it wouldn't cause his supporters to think again about backing his policy.

The same runs for excessive trade retaliation. Small, measured counter-tariffs — of the kind already promised — are a political necessity for Europe and another targets of Trump's moves, because those governments can't afford to be seen as capitulating. But going beyond those mild responses and deliberately intensifying the fight is unlikely to work.

In public, the six governments( together with the EU) should make it as easy as possible for Trump to back down. In private, though, they should increase the pressure on him to do so by showing they will act in a coordinated style, both in designing their responses to this first stage of protection and in replying to any subsequent measures. Trump proposes to divide and conquer — a strategy once reserved for foes. The G-6 should refuse to be divided.

Another thing to bear in mind: Time is on the side of the G-6. Opposition to Trump's tariffs from U.S. business and consumers is likely to mount as their effects become clear. If Trump persists, taxes on trade will push up costs for U.S. consumers and disrupt the operations of the country's internationally integrated companies; they'll slacken the economic expansion and the growth of aggregate employment; firms will contract and workers will lose their jobs.

The best hope for smarter policies from the White House is that more Americans begin to see this, demand change, and press that view on a Congress that has been too reluctant to act.