One of our candidates for president has said it outright, and Republicans whine about helping Ukraine at every opportunity. If I were European, I wouldn’t be sure I could count on the U.S., either.
The alternative is leaving them to get “special operationed” without any outside help, and then Poland or whoever Putin thinks he deserves to take land from is next, isn’t it?
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.
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.