During the cold war, the US and Russia, along with minor contributions from Europe and China, learned to use their nuclear arsenals to prevent nuclear war. It was a thing nobody wanted - if you can't win, what's the point in even playing? So elaborate signaling and systems grew up, even as systems evolved to the incredibly dangerous "Launch on Warning" standard we still carry today. But even so, when something went pear-shaped - and it did fairly often - they were able to avoid the most obvious, and horrific, outcome.
The problem is, as technology advanced, the window for decision making got shorter, and we're now faced with a condition where it would be very easy for the situation to outpace our ability to manage it. And yet, the cold-war logic holds even today, if we operate within its constraints. Here's what you can't do - you can't build a system that takes you out of the 'MAD' - Mutually Assured Destruction - construct. As long as we sink or swim together, we can work together to prevent the use of strategic nuclear weapons. The situation is destabilized, however, when one side or the other has a qualitative advantage that would permit it to launch an effective preemptive strike that effectively 'wins' a nuclear exchange by preventing substantial retaliatory strikes by the targeted country.
There are exactly two ways you can do this - using anti-missile technology to render your nation invulnerable to ICBMs or to develop a weapon/delivery platform combination that would allow you to eliminate your opponents strategic capacity without warning. This combination of stealth, accuracy and weapon effectiveness is much more likely to find itself in play than some kind of imaginary missile defense system.
Which brings us to the US Nuclear Weapons Policy. The Obama administration has affirmed that they will not develop 'new' weapons, but just where the line between updates and modifications to existing designs and entirely new weapons is can be difficult to agree on. A nuclear weapon is comprised of the nuclear explosive element itself, called the 'physics package', the mechanisms that render it safe until they actually detonate it, and the 'packaging', the functional bomb or missile itself. If you retain the same physics package, but you add modern digital control circuitry and an advanced guidance and delivery package, is that a new weapon, or just an upgrade to an existing weapon?
That is the effective description of the B61-12. The US has had the B61 thermonuclear gravity bomb in inventory for decades. It's always had a few key features - variable yield technology, and limited accuracy. The B61-11, fielded in 1997, was built in a ground penetrator 'bunker buster' configuration. The B61-12 is the same 'physics package', but packaged in a modern precision-guided gravity bomb and deliverable from stealth aircraft like the B-2 and the F-35. This combination of stealth, precision and low yield make it ideal for decapitation and anti-nuclear first strikes, which makes it inherently destabilizing. When the Russians have to realize that the first they would know about a US first strike would be the destruction of the Kremlin or their missile silos, their finger is going to be that much tighter on their own nuclear trigger.
So we have to ask: Why? Why do US forces see a need to turn a 1970s era gravity bomb into the most destabilizing nuclear weapons system in the world? What problem are they solving with the B61-12? The answer is as obvious as it is frightening. They are working not to preserve a strategic 'balance of terror', but rather their own budgets. They have to walk a fine line, being able to plausibly claim these are mere upgrades of existing weapons, but the only way they can deliver an upgrade is to turn a city-killer into a silo-killer. And make no mistake, the Russians and Chinese are watching very closely.