The US hasn’t been isolationist since WW2, quite the opposite once we were relatively unscathed by the conflict and rose to the worlds #1 military superpower.
"Since"? US was isolationist for a large part of the war. It's their underlying policy. Sure they do sometimes deploy to get oil from the middle east and such, but that is quite different. As was the whole libya thing, concerned about the gold dinar, not some leader figure.
It seems to me Trump is following the logic that the current danger to the US is China, not Russia. Approaching Russia may be a strategy to ensure China and Russia (and maybe all BRICS) are not aligned against the US. He probably believes that Europe is not a threat, and won't become one even when the US behave in ways that go against its interests (Which I would say is correct).
I have to say that while that world view may be misguided, and certainly is not a worldview Europeans would agree with, it is nonetheless a rational view, and is almost certainly correct in that Russia alone is not a serious threat to the US, and won't be in the medium term at least (it can barely win in Ukraine, to think it could win against NATO and then go on to take the US is just delusional).
