His smarmier comments are bad enough, but Trump's suggesting the US might renege on our NATO commitments is both an immediate and long-term problem.
I'm one of those people who doesn't see ISIS as an existential threat to the USA and world stability but the harbinger of successor groups that could get widespread support, find a charismatic leader or two, and get us back to the days of warring mass movements. Christian Europe in the 16th and 17th centuries showed what could be done with wars of religion in a time of a revolution in communication (cheap printing) and others in military affairs. A Sunni/Shia civil war combined with an incipient Caliphate taking on a crusading Christendom in the midst of climate change — would not be good. So "ISIS must be destroyed" before it evolves and further metastasizes — and nuclear proliferation needs to be stopped and the number of warheads further reduced: and that means US cooperation with the Russians and the Iranians, including some accommodations with Russian concerns about its "Near Abroad," which is much of Eastern Europe and the Baltic states.
So only recently freed members of "the prison-house of nations" of the Russian Empire (and the Israelis) are going to be very, very nervous and need to be confident that a US/Russia "reset" doesn't include throwing them off the troika to the Russian wolves and bear. And handling *that* problem makes it absolutely essential that Estonia, Latvia, Poland, and the rest are totally confident that there is a US guarantee of their sovereignty that is non-negotiable, and the principle of "An attack on one is an attack on all" is crucial to that confidence.
"The Art of the Deal" with the Russians can't include a US President who has already given their fondest desires on NATO to Russian expansionist jingoes, nostalgic for the glory-days of Stalin and the Empire, and the fight for Mother Russia and the Orthodox Church against the Infidels. Trump, Clinton, and indeed the Democrats and the rest of the political class need to get serious about dealing with Russia as a vast country with whom we will compete and must cooperate.