As a European, I'd say it is very much in America's MO to come into a conflict all guns blazing and with lots of bluster, then get caught in a quagmire for an absurdly long time, then eventually randomly declare that they've won and just bail, with very little regard for whoever's left still fighting who isn't American.

It seems to happen with them pretty regularly, and for an example it was only about three years ago that they hurriedly pulled out of Afghanistan and abandoned the local translators to their fates with the Taliban.

With regards to Russia encroaching into Europe, could we be 100% certain that Trump won't win the next election and start throwing favours to Putin? I'd say it's certainly a possibility.