So, Russia apparently continues down a path towards re-instituting a large part of it's communist past, South America is increasingly shifting far left towards a brand of socialism thought dead by most reasonable people and Moslem jihad has sprouts in Asia and Europe.
There is almost no way that either Russia or South America would join in a Moslem uprising world wide, but they could join forces. If perchance some type of union of Russian socialism joined with South American socialism, might the jihadist take advantage of a distracted United States and promote a general uprising? There is no evidence that I have seen or heard that such a call would generate anything approaching dangerous - at least not for the majority of people, but what if Europe/NATO called for American reinforcements?
Why should we answer? Is there any reason right now why the United States should honor any previous commitment for troops or material support should a military response be required in Europe?
My general answer is found above. Honor. Over the last 5 years we have found which countries have honor and which do not. Despite the overwelming evidence that most of Western Europe lacks even the basic understanding of the term, WE would honor our commitments.