The presence of nuclear self defense is what keeps nuclear armed nations from engaging in aggressive wars with other nuclear armed nations.
The US and Russia are already in the midst of
several proxy wars: Ukraine, Syria and Libya. In addition, Russia is directly destabilizing Europe by funding alt-right parties financially (AfD/DE, FrontNational/FR, FPÖ/Austria, Jobbik/HU, Lega Nord/IT) and by promoting their crap on Twitter and Facebook, as well as their "media outlets" that historically have spread alt-right propaganda (Sputnik, RT), but now have expanded to liberal/leftie propaganda (Inthenow, Ruptly, Redfish).
The Chinese seem to be playing the long game by strategically investing in infrastructure like roads and rail across Africa, probably both for strategic advantages (given Africa's large reserves of anything from rhino horns over diamonds and uranium to rare-earth metals) and economic advantages (the US and EU markets are saturated, South America is going down rapidly, but Africa can only grow and buy Chinese products in return - basically the Marshall Plan just without the war).
Also, both China and Russia engage in cyber warfare - the Chinese right now mostly for economic espionage, the Russians at the moment for destabilizing democracy.
What worries me is the shit some Arab countries are trying to play with Israel (esp. Turkey!) and the skirmishes between India and Pakistan, as well as Pakistan's problems with terrorism. Both countries are highly volatile and hell I don't want to experience the Taliban getting hands on Pakistani nukes.
So, there are LOTS of possibilities for nuclear-armed powers to go to war, and some not even violent. The key is that no power ever actively uses nuclear weapons, but there's nothing that prevents them from using conventional ammo. There's enough of that alone to blow the planet to pieces.