Last week, the Trump administration used Section 232 of the Trade Expansion Act of 1965, and cited national security as a rationale for the punitive measures imposed on aluminum and steel imports from Europe and Canada. The targets of the tariffs bristled at the announcement not only because they were the principal targets of the levies, but also because of this cited rationale. Canadian Prime Minister Justin Trudeau called it "an affront to the ... thousands of Canadians who have fought and died alongside American comrades-in-arms."

If there were doubts whether the U.S. action would be confined to steel and aluminum imports, news reports said Trump was considering using Section 232 to limit the number of German luxury cars entering the U.S. Joshua Meltzer, a former Australian diplomat who is now a senior fellow at the Brookings Institution, said on a conference call with reporters that the strong reaction from the G7 was to head off further use of Section 232.

"There is a very thin national security justification for doing this for steel and aluminum. I think it's very, very flimsy, but it's not completely absent," he said. "I think once you look at extending that to autos, you've essentially thrown out any pretense that you're actually doing this for genuine national security interests. You're clearly into a protectionist world. Given that national security is an exception embedded into all of the international trade rules, you've essentially blown that wide open."

The disputes over trade loom over the meeting among the G7 leaders that begins Friday. That is not to say, of course, that previous G7 meetings were free of tensions: Since the first summit in 1975, there have been divisions over inflation, U.S. missiles in Europe, U.S. budget deficits, and the war in Iraq. What's different now, however, is how America's allies perceive how the Trump administration views the rest of the world.

"While there have been differences in the past over specific economic policies and specific issues, there's never really been a fundamental question about commitment to the post-war international economic order," Thomas Wright, who is the director of the Center on the United States and Europe at Brookings, said on the same call. "That's really what's in question today. We do seem to be a little bit in a Rubicon moment in terms of whether or not the Unites States is going to fundamentally turn its back on this economic order."

Speaking to reporters Wednesday, Larry Kudlow, who is director of Trump's National Economic Council, said: "The world trading system is a mess. It has broken down." He compared the current situation within the G7 as a "family quarrel" that can be worked out, but he added: "The president is very clear with respect to his trade-reform efforts that we will do what is necessary for the the United States, its businesses, and its workforce [and] tariffs are a tool in that effort."