I guess it would be bad if the interceptors were highly concentrated in a single nation or coalition, but as long as each coalition has plenty (relative to the number of warheads) it shouldn't be a problem. Note that the distribution doesn't even need to be very even--let's say each nation is capped at 100 warheads and Russia has 1000 interceptors and the US has 10000--the US doesn't have much leverage on Russia despite an order of magnitude more interceptors. So as long as all of the interceptors aren't concentrated on one side of an engagement (and it would be very unlikely for this to be the case for any enduring period of time) all is well.
And of course, there's the additional deterrent of a globalized economy. Most nuclear countries depend on the global economy and have a strong incentive not to devastate it by way of nuclear warfare. The odds of an isolationist country like NK owning all of the interceptors is pretty low IMO.
> Increased counterparty desire to build more ICBMs
I'm assuming a cap on ICBMs. If nations aren't going to obey these caps then why should they obey a cap on interceptors?
> Increased belief in a successful first strike / acceptable response casualties
It doesn't follow that (num_interceptors > num_counterparty_ICBMs) will lead to a successful first strike (notably it doesn't follow that the counterparty doesn't have interceptors of their own).
Arms control treaties are negotiated in the current context, in country's best interests.
Even a desire to build more ICBMs is harmful, as it makes a country less likely to voluntarily agree to ICBM limits in the future. But absolutely agree that ICBM and missile defense are intertwined, so only make sense to negotiate strategic arms control covering both.
I would say that it does follow that "sufficient" interceptors, relative to a counterparty's ICBMs, leads to a "successful" first strike.
Where "success" in this calculus means acceptable losses.
The lynchpin of MAD is that no one is insane enough to argue that the incineration of a major city is acceptable for a first strike.
Without ballistic missile defense, that destruction is effectively guaranteed, as long as the counterparty has available ICBMs and is able to launch them.
With ballistic missile defense, there's a chance that destruction could be averted. And that chance drastically balloons the risk of a first strike being launched.
To put it another way: you're the president, and you're at a meeting of the Joint Chiefs. Open conflict with Russia has broken out in Eastern Europe. Massive amounts of Russian reinforcements are transporting towards NATO lines.
No BMD: if we escalate to nuclear, and the Russians retaliate (as stated by their nuclear posture), American cities have a 100% chance of being vaporized.
BMD: ... American cities have a 75% chance of being vaporized.
That's a terrifyingly different decision, because it opens the possibility that someone decides to take a risk. Because there's a chance things will work out favorably.
I guess I'm arguing that there is a dynamic you're not considering with respect to whether or not to initiate a nuclear strike. I agree with you that an aggressor is more likely to initiate a strike if they believe they have a very high chance of a successful strike (where "success" is defined as destroying the target) and a very low chance of retaliation. However, the scenario we're debating is one in which Russia has its own interceptors so the risk of a successful first strike (US striking Russia) is about the same as the likelihood Russia succeeds in retaliating.
If both nations have lots and lots of interceptors relative to the ICMB cap (assuming for the moment that both countries adhere to the ICMB cap), then the odds of any country initiating a successful strike (first or second) becomes relatively and equally low. At that point, it's more effective to pursue conventional (i.e. non-nuclear) warfare, and even if a country does initiate a desperate nuclear strike, it's less concerning because the odds of a catastrophic loss of life are significantly reduced.
Would you argue that we would be better off if we fought conventional wars without body armor of any kind? Or would you argue that this is somehow different than the interceptor question? If so, how is it different (degree vs kind and why)?
(By the way, I don't see a US president--barring perhaps the current one who doesn't seem concerned with his own best interest much less that of his country--accepting a 75% chance of the destruction of US cities in order to stop a Russian advance)
Good points, although the decision does bring in the anti-ABM research progression as well.
In that, if I am facing an adversary with sufficient ABM capability, I am inventivized to try technologies that moot that advantage. E.g. nuclear kamikaze UUVs/smart torpedos, stealth cruise missiles, short range/quick reaction strikes etc.
I think non-parity relationships are somewhat alien to us today, whereas the 50s and 60s were full of "someone could nuke someone else with impunity" periods.
And I guess I look at it as: MAD is preferable to a Pandora's Box future where sides are continually researching, developing, and deploying new weapons and reevaluating their relative advantage. Advantage in an arms race being especially troubling because it's use-it-or-lose-it -- e.g. the internal calls for the US to use nukes against China before they developed their own during the Korean War.
> Would you argue that we would be better off if we fought conventional wars without body armor of any kind?
It's absolutely a related question.
To some degree, yes. Or a corollary in whether it's just to fight wars in which one side uses drones/UVs and the other side uses people.
Modern wars end when one side's will to continue it ceases. And, for better or worse, casualties have driven that. What happens when one side doesn't take casualties (e.g. Yemen)?
And of course, there's the additional deterrent of a globalized economy. Most nuclear countries depend on the global economy and have a strong incentive not to devastate it by way of nuclear warfare. The odds of an isolationist country like NK owning all of the interceptors is pretty low IMO.