I don't agree with the execution, but here's what I think this is what Trump is really trying to do:
Right now, it's basically the US vs Russia and China.
But what Trump is betting on is that it could be the US + EU + Russia vs China.
In order for this to happen though, the EU must build their own military.
The EU could and should be a superpower, but they're not. Instead they're living off the US military.
The US has been gently trying to get EU to boost its own spending. This goes as far back as Obama, and maybe even further back.
The difference is that Obama was somewhat passive aggressive with his approach. Whereas Trump is tearing off the band aid.
The public belligerence and positioning against the EU is an effort to kick them out of the parents' basement and get their act together.
Now, what is he doing regarding Canada and Mexico? That I have 0 clue. And his current stance honestly seems stupid.