A White House spokesperson put it the best, I think:
The White House spokesperson Andrew Bates, when asked about Trump’s comments, said: “Encouraging invasions of our closest allies by murderous regimes is appalling and unhinged – and it endangers American national security, global stability and our economy at home.”
What’s so insane about all this is that Trump saying something like that out loud means it’s now practically gospel to the vast majority of GOP voters, and might as well be official party policy.
Well, he obviously shouldn’t.
He has to say that, though. Saying “if Trump wins, Nato is hosed” wouldn’t sound as, well… resolute