I don't pretend to know much about the economics behind this, or even fully understand how tariffs and a trade war will impact business and the U.S. economy somewhere down the line. What I am beginning to suspect is that perhaps Trump doesn't, either. He has made decisions that he then pulled back after learning that it was an "Oops" move, and he has done his typical "nice to their face and then insulting on Twitter" move here, as well.
My guess is that behind the scenes, Trump has these ideas about how business should be run. The fact that he has done business around the world, successfully, for years does not mean he understands how the global economy actually works, but the people who DO know how it works can't get through to the President, so he comes out with his fiats and then after enough people say "See, this is what I meant.." he realizes it's a bit more complicated than he realized. Kinda like healthcare was.
Trump's supporters don't seem to understand that being a successful international businessman doesn't mean you know as much as you think you do. Who knows if there's any real or lasting damage to the US economy because of the bruised feelings from the G-7 summit. Those leaders have to object to policies that would interfere with their economies--but it may be a lot of posturing. And once again, Trump may soften his position. This art of the deal shit, being done on the public stage, has everyone in an uproar, but I'm not convinced it's as awful as people are making it. The trade war would be, though, and let's hope it doesn't come to that.