Um, Trump would have told Russia to just go right ahead, I don't want them as part of NATO and I don't want to pay NATO's bills anyways.
Here is what I don't get about the "Putin didn't invade under Trump because Trump was nice to him" claims.
The only legitimate complaints about Trump's dealing with Russia is what he said. When it came to action, he was very hard on Russia.
So if all it took to stop Putin from invading was saying a couple of nice things about him (without giving him anything of substance), then maybe that's what we should be doing.