Russia and NATO tensions have been there since the inception and U.S. back-peddling in the expansion of NATO even though the Soviet Union has fallen has further increased these tensions (i.e. U.S./NATO arguing it would not further expand). I'm not sure if I would argue Syria is increased NATO tension; rather, U.S. tension whom runs NATO for all intent and purpose. NATO is there to protect all the smaller states and if anything, interior tension has increased due to the Russian annexation of Crimea with very little U.S. response.
None of this relates to the Cuban Missile Crisis unless you are referring to the tension during said period and if you are, they are no where near as close. The U.S. had nuclear weaponry off the shore of the U.S. A proxy-war is nothing compared to that and the U.S. has been exchanged in them previously (i.e. Afghanistan).
Modern weaponry has shifted away from mass-destruction and to that of pinpoint attacks. There is the nuclear deterrent and the world has yet to see two nuclear powers have a major conflict with one-another and even if they did come into contact, I would not expect nuclear weaponry to be used on any wide-spread basis (pin-point if anything).
I think people significantly underestimate the global factors that come into play in the modern world. A world war would wreck the global economy and there is not currently any country that stands a chance against the U.S. in actual military conflict. The U.S. is not just a hegemonic power, it is a global power, and it is the sole super power. It is a unipolar system and the U.S. is currently running the show. The U.S. defense budget is larger than the next 8 highest spenders combined (majority of which are very close with the U.S.) and while China is stepping up, it is doing so in a hegemonic way. China wants control of it's own region - much like the U.S. initially wanted with the Monroe Doctrine.
