That we know of 

 I've heard multiple times that Russia has nuclear subs in the ocean off the East Coast of the US and if a nuclear exchange were to eventuate, their sub-launched missiles would hit rather rapidly.  And that the US has similar close-range strike capability from Europe.

My point was that we have been silently in close strike range for decades(?) and this has been normalized. Back in the "Cuban Missile Crisis" such close strike range was very worrying to Americans who didn't know it was mutual and Kennedy secretly made a deal to pull our missles out of Italy and Turkey.  But even at that time, it wasn't a real "crisis" because it was mutual and so neither side could win, and neither side was going to push the button.

Russia being at Cuba today doesn't present much extra threat of a nuclear exchange. The US intel agencies have already publicly said there are no nuclear weapons on those Russian ships.  What it does present is a conventional attack platform with Kinzhal and other hypersonics.  It is meant as a warning, since the US keeps crossing red lines:  "cross another red line attacking Russia with your missile systems and perhaps we will attack the continential USA with our missile systems... we are serious, stop pushing us."  It is meant to stop the escalation cycle.  "Escalate to deescalate".  But of course every escalation could provoke more escalation in retaliation instead. 

 My understanding is that one of Russia's concerns with the expansion of NATO into Eastern Europe is the US deployment of anti-ballistic missile systems in countries like Poland.  The US wants to be able to do boost-phase or mid-flight intercept on Russian ICBMs.  From Russia's perspective, that upsets the strategic balance of power and gives NATO the upper hand in a first-strike nuclear exchange.

I wonder if the deployment to Cuba is meant to play into American politics—we are in an election year.  In WWII a strike on American soil fired up the people to support the war effort.  Today, a similar attack might just demoralize the American populace and make any war immediately unpopular.  We're so used to all the fighting happening "over there," and patriotism seems to be low. 

 Pepe Escobar says Russian leadership and businessmen are pretty sure NATO intends to wage a full scale war on Russia, that their moves in Ukraine are a prelude, a means of weakening Russia first.  So Russian strategy has been with this in mind:  their rapid alliance with China, their conversion of their economy into building ammunition, their strategic escalations, all with the view that war with NATO is inevitable.  Most of them want to decapitate Ukraine and get it over with, so that proximal axis of war would be cut off and NATO would need to come up with a different excuse to go to war with Russia.  But Putin has been patient and hasn't taken such an action yet as far as we can tell, instead focusing on setting up meat grinders and winning by attrition.  A decapitation action that wasn't an assassination might take about 3-4 months (according to former leader Sergei) of attacking Kiev. 

 The pattern I seen to be seeing in NATO is that they're looking at Ukraine and getting a feel for how a real present-day war would be fought (and not just bomb-dropping in the Mideast sandbox).  They're seeing the sheer volume of ammunition expenditure and going "oh shit, we need more bombs, guns, and missiles."

So now NATO, and the US in particular, is trying to figure out how to more rapidly build and maintain stocks of war materiel.  The problem is, that will take years, and there might not be that much time before, say, China makes a move on Taiwan.