Not to nuke the US, but to take its balance on the world stage, so it can be thrown easily to the mat if and when the time comes. Putin is a Judoka. He thinks like a Judoka. This is evident in all of his playing of the Great Game. He is slowly but surely fucking up the credibility and internal cohesion of the West, but specifically the US. He wants to reach a point where it is beyond certain he can reclaim territories in Eastern Europe and the Balkans without an appropriate NATO response. Helping NK provoke a war with the US fits perfectly into this strategy. Of course he is going to denounce on the surface, but if he was really against NK having nukes, Spetznaz would've put two in Kim Jong Un's brain a long time ago. This situation is to Russia's advantage, and even moreso if tensions escalate into a hot war. China, of course, is watching from the sidelines. The ideal situation for them would be for the US and Russia to engage in a full-scale conflict themselves, and wipe each other out, leaving China as the sole superpower.