So, here's who WW3 will unfold, about 20 years from now. Once the USAF can mount laser weapons on a plane, not long after they'll be able to put them in missiles. And more precisely in those missiles that perform the anti-ballistic missile defense function. Currently the equation governing the ABM defense is that you need about 2 kinetic vehicles to destroy one warhead launched at you. If you replace this with one laser-equipped missile can zap 10 incoming targets, that's a complete different ballgame. The MAD concept can become very unstable. Instead of needing a many, many thousands of missiles to obtain impenetrable defense, two hundred missiles-mounted lasers could do the job (a single Ohio class submarine can carry up to 24 Trident-II SLBM's with up to 288 multiple independent re-entry vehicles; if you replace each such nuke with a laser, you are all set).

When will the US have this capability? I would ball-park this at about 15 years for now.

What options does Russia have. By my cont 3 options (hardware-based; I'm not counting cyber-warfare and psychological and subversive warfare).

1. Deterrence in numbers. Currently Russia deploys 1600 warheads, and has a total of 6500 (deployed+stockpiled+retired). If they go to 20-30k warheads, in an all out war a few dozens might still find their way to some large urban areas in the US, and so even a 99.9% ABM capability might still leave a few tens of millions of Americans dead.

2. Laser-equipped anti-anti-missiles. If you can zap a nuke out of the sky, you can zap the missile carrying the laser too. Provided you have the technology. I expect Russia to have a delay in implementing this technology, maybe 5 to 10 years

3. Alternate threats that are immune to this defense. This is exactly what Russia demonstrated one year ago: a hypersonic nuclear-capable, nuclear-powered missile that can travel in the lower atmosphere (where the effective range of a laser is reduced, due to light absorption) plus a mega-nuke delivered by an autonomous submarine that purportedly can create a 500-foot tsunami that would wipe NYC from the map.

Where does that leave us? The US will enjoy the maximum military gap about 20 years from now and this will provide a huge incentive to perform a decapitation strike. Would this be palatable to the US population? Only if it can be demonstrated that the collateral damage would be insignificant. Is that possible? What do you need to do? You need to take out the missile silos, the strategic nuclear submarines, and the strategic bombers (the nuclear triad). The silos are traditionally the targets of some nukes; in the past these nukes had about one megaton yield, but with the increase in missile accuracy the current US yield is about 100kt; this is still 5 times larger than Nagasaki. Is it possible to use a smaller nuke with an increased accuracy? A rule of thumb is that a 10-fold increase in accuracy can reduce the necessary effective yield by 100. Starting this year (2019) the US Navy will have a 5-7 kt nuke in its arsenal (W76-2 [1]). This type of nuke could ensure essentially zero civilian casualties, while still being able to destroy most missile silos. Oh, and it would also be a good anti-strategic-submarine weapon too.

So, about 20 years from now, the US will have a once-in-a-century opportunity to break the mutually-assured-destruction stalemate, and potentially rid the world of nuclear weapons. All with minimal loss of life (especially civilian life). But the window of opportunity will be very short, Russia will be able to close the MAD gap in a few years. Which will make this opportunity a very difficult one to pass on.

[1] https://www.theguardian.com/world/2019/jan/28/us-nuclear-wea...