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Apologies if this is posted in the wrong place, but I couldn't find "military" stack exchange so this site appears to be the closest.

While following the current Russia/Ukraine war, it appears that a considerable amount of (conventional) missiles that get fired are shot down by the defenders' air-defence systems. I've also heard of Israel's "Iron Dome" which is capable of intercepting incoming missiles.

What does this mean for the policy of Mutually Assured Destruction? Does it still exist? It would seem that in a "second strike" scenario, many of the missiles could be shot down before they reach their targets, and since even the larger nuclear powers have a limited number of weapons, how can they be sure their retaliatory strike won't simply be snuffed out by their enemy's air defence?

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  • Since you seem to be asking about the practicality, not the philosophy, How is this about Politics rather than something military? Either way, why are you worrying about retaliatory second strikes, when your thesis seems to be that the first strike won't work? Nov 16, 2023 at 23:40

8 Answers 8

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ICBMs are notoriously hard to intercept. In their final flight phase (where normal anti-missile systems intercept), they are very fast, usually split into many different targets, and have countermeasures against interception. Normal anti-missile systems just wont do the job since they lack range, detection capabilities, and/or speed (cheers to @DavidS in comments) for targets coming in from this angle at that speed.

This results in only six states having the ability to intercept ICBMs at all, but those systems that are deployed are tested only against single (or max 2) ICBM-like test objects (maybe with inferior decoys as additional targets) and it is usually assumed that any large-scale attack with ICBMs will not only be met with unreliable interception systems but that they will also be easily overwhelmed by the sheer number, speed, and ability of targets.

Therefore, even the most sophisticated systems of today will not be able to effectively neutralise attacks of even minor nuclear powers like Iran and North Korea.

The second link below argues that this will be the case for at least another 15 years for the US with no signs of other systems faring any better. This view was contested by officials who nevertheless admitted that there have never been tests with more than two ICBM targets at a time

Sources:

https://en.m.wikipedia.org/wiki/Intercontinental_ballistic_missile

https://ww2.aip.org/fyi/2022/physicists-argue-us-icbm-defenses-are-unreliable

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    That APS study was written by well known advocates. It's IMHO no more reliable than MDA's word to the contrary. breakingdefense.com/2022/02/… Assumptions about the sophistication of NK ICBMs, especially by those apparently w/o security clearance are just that basically, assumptions. Nov 14, 2023 at 14:36
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    OTOH Chinese scientists write papers in which they assume that their ICBMs will be intercepted by the US. See the last para in my answer to a related Q: politics.stackexchange.com/a/69440/18373 Or here's a more direct link doi.org/10.1162/isec_a_00376 Nov 14, 2023 at 14:40
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    Another thing we leaned in Ukraine: each Kinzhal/Iskander apparently drop 6 decoys thedrive.com/the-war-zone/44760/… And they're active decoys, both radar and IR. Nov 14, 2023 at 19:30
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    @NoDataDumpNoContribution If we had a military stack, your question would be a good one to ask in a different manner. Who found out? More or less everyone with some understanding of missile systems. Its a very very broad question to answer the how part. You can start here: "In general short-range tactical ABMs cannot intercept ICBMs, even if within range. The tactical ABM radar and performance characteristics do not allow it, as an incoming ICBM warhead moves much faster than a tactical missile warhead." en.wikipedia.org/wiki/United_States_national_missile_defense
    – David S
    Nov 14, 2023 at 23:42
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    @DavidS What you cite in a comment is what I would expect in the answer. I mean that the statement "wont do the job" is really important and everything the answer hinges upon. If the current systems at least partly would do the job, there would be a problem with the logic here. I agree it's a technical part and should be answered elsewhere but even here we have to fact check our statements at least a little bit. Or we simply reduce our statements to what (defense) politicians think about the matter. Citations there would also be welcome. Nov 15, 2023 at 7:25
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It would seem that in a "second strike" scenario, many of the missiles could be shot down before they reach their targets, and since even the larger nuclear powers have a limited number of weapons, how can they be sure their retaliatory strike won't simply be snuffed out by their enemy's air defence?

Russia's use of missiles in their current campaign is not particularly similar to how missiles are envisioned as being used in a nuclear "second strike". There's no reason to assume that defenses that are capable against one will be equally capable against the other.

There are three key differences that stand out to me:

  • Flight pattern. Cruise missiles and regional ballistic missiles fly lower and slower than ICBMs. The longer a missile spends near the ground, within the engagement envelope of air defenses, the easier it is to shoot down. A higher, faster missile with a sharper terminal phase is harder to hit.

  • Distribution of targets and launchers. Air-defense is very regional. Assets that are well-positioned to protect one avenue of attack are useless if the attack comes from somewhere else. In the current Russian campaign, the launch sites and targets are usually pretty predictable. In all-out nuclear war, the launch site can be almost anywhere that a nuclear submarine can loiter, and the targets are your entire country.

  • Quantity of missiles. Air defenses are very sensitive to volume of incoming fire. Conventional missiles are a precious resource you need to ration for the rest of the campaign, and for use against other enemies, or to respond to emerging threats. Even with a stockpile of hundreds, it's hard to free up more than a handful for any given attack. With ICBMs, there's no use saving them for a rainy day because this is It. If you have hundreds, you launch hundreds.

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  • "A higher, faster missile with a sharper terminal phase is harder to hit." Sure but one would have a lot of flight path tracking before which means some warning time and say a modern Patriot system can probably go as high as, dunno 20km? And the numbers of nuclear warheads would be lower then with conventional missiles. The question is probably how many percent could realistically be intercepted. Nov 14, 2023 at 17:50
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    @NoDataDumpNoContribution "And the numbers of nuclear warheads would be lower then with conventional missiles." Depends on who your adversary is. If it's North Korea, yes. If it's the U.S. or Russia, very, very no. Both the U.S. and Russia have thousands of nuclear warheads. In the case of all-out nuclear war, most of those would be launching more-or-less simultaneously and a very bad day will be had by all involved.
    – reirab
    Nov 15, 2023 at 0:10
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Mutually assured destruction has never required all of the warheads make it to your enemy... or even that most of them do. In fact it is the exact opposite. Military planners have assumed since the late 40's and early 50's that most warheads would not arrive, but MAD still operates even in such a situation.

The name for this phenomena was originally, "the bomber will always get through," and then later, "the missile will always get through." Shortly after nuclear weapons were developed, the next question was how to defend against nuclear weapons. The short answer is that you cannot... or rather, you cannot effectively defend against a nuclear strike to the point where launching your own strike seems feasible. The basic reality is that nuclear weapons are so incredibly destructive that even a single bomb serves as a credible deterrent.

Suppose you want to launch a first-strike... is it worth it? The thought process goes like this:

  • Suppose you can destroy half of your opponent's weapons in a first strike
  • Then half of the remaining fail to launch
  • Then half of the remaining are shot down by missile defense
  • Then half of the remaining fail to land on-target
  • Then half of the remaining fail to function
  • The remainder detonate successfully.

At the bottom of the list, only one out of every 32 adversary warheads successfully detonates on target, or only a 3% success rate. If your opponent has 100 nuclear weapons, then that means three bombs succeed and make it through. A single nuclear bomb, especially by late 50's and into the 60's and certainly today, is more than capable of functionally destroying a city.

So here's the political calculus. Suppose you're President Kennedy- you can launch a first strike and obliterate the entire Soviet Union, but three of their weapons make it through and land on three American cities. Is that an acceptable exchange? Suppose you get to destroy the entire USSR, but Washington DC, New York, and Los Angeles are turned into smoking craters- three of the most populous and influential cities in the nation and the world. Are the American people going to stand up with you and applaud you for a job well done? Are you sure about that? What about all the people who had friends or relatives living in those cities? What about all the businesses that had clients in those cities? What about all the people who depended on those cities for services? Then add into that all of the anti-war folks, peaceniks, environmentalists, and anyone else who doesn't like the idea of lighting off thousands of nuclear weapons. When you have to run for election again, are any of those people going to have second thoughts about you in the voting booth?

Maybe some people would gladly trade grandma and their livelihood to see the USSR reduced to ruin, but most people won't. Or at least as a politician, you can't be sure either way. Any nuclear exchange could easily become political suicide, even if your opponent's weapons only have a 3% success rate.

And this is the reality of nuclear weapon defense. Nuclear defense only works if you can guarantee your system works 100% of the time, or maybe 99.9% of the time. It does not work if it only works 97% of the time, or 95% of the time, or 90% of the time, because your opponent has hundreds or thousands of warheads, and that could be three, six, a dozen or more cities turned into smoking craters.

In reality, under practical scenarios, nuclear missile defense has nowhere near a 50% success rate. If for no other reason than a lack of available interceptors. In the USA, the mid-course interception system, or GBMD (Ground-Based Midcourse Defense), is the only anti-ballistic-missile technology capable of defeating the large strategic ICBMs that would carry nuclear warheads from Russia or China to the USA. There are only 44 GBMD interceptors available. Even if every single interceptor was successful (which is not likely), the major nuclear threats to the USA have hundreds of warheads and launch vehicles.

If any of the assumptions in the bullet-point list above are not true, then your adversary is not just landing three weapons on your country, they are landing dozens or hundreds. If you only have 44 interceptors and they launch 300 separate missiles, then hundreds of missiles are getting through. If you thought only half of your opponent's missiles would function, but instead 95% of them function, then hundreds of missiles are getting through.

The other US anti-ballistic-missile systems such as THAAD or Aegis are not designed to be used against large intercontinental ballistic missiles (they are designed to intercept short- and medium-range missiles), and while US military forces are prepared to employ these systems in such a scenario, that's really a last-ditch effort and it is not expected to be effective.

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it appears that a considerable amount of (conventional) missiles that get fired are shot down by the defenders' air-defence systems. I've also heard of Israel's "Iron Dome" which is capable of intercepting incoming missiles.

In order to shoot down a missile, your interceptor has to reach it. It is considerably easier to do so if the enemy missile cooperates by coming straight towards the anti-missile site. This leads to two scenarios:

  1. Target is tiny (aircraft carrier, single city, Israel...) and it can reasonably be expected that it will be attacked with missiles.

In this case the expense of a defense system is justified, and it can be placed in the location where it will be most effective, which is very close to the target.

Since the enemy missile is coming at you, your interceptor doesn't need to have very long range, it doesn't have to catch up with the enemy missile, so it can be smaller and cheaper, which means you can have a lot more of them.

These are reasonably effective. For example the US Aegis system uses various types of anti ballistic missiles which are expected to perform adequately if China were to fire conventional (non-nuke) ballistic missiles at them. Of course, if the enemy has more missiles than you do, or can convince you to shoot at cheap decoys so you exhaust your supply, then it doesn't work.

  1. Huge area (USA, Europe...) with a large number of potential targets.

In this case, you don't know where the missile will be aimed at. You don't even know where they will be coming from, because nuclear subs could be anywhere in the sea. So you need to put anti-missile missiles everywhere. There's a compromise on range vs cost, but you get the idea. It's a completely different problem from the previous one. Besides, with MIRV nukes you need to fire many interceptors to make sure you get all the warheads, which increases the cost dramatically.

In fact, unlike the previous case, there probably won't be any nuclear missiles coming, so it will be difficult to justify this prohibitively expensive system.

Chances of losing the next election would be high.

Summary: in your question you are comparing two things that are not really comparable.

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Mutual Destruction is certainly less "assured" nowadays.

The idea of MAD, however, still exists. At least somewhat. Because some nuclear countries don't have the best air-defence systems, especialy the bad players on the arena (those who care more to destroy others than to protect their own citizens). In addition, no currently available Air-defence system is 100% guaranteeable.

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  • "no currently available Air-defence system is 100% guaranteeable." - true, but surely, if you maintain a nuclear deterrent you want something more definite than "their air-defence system isn't 100% guaranteeable". Especially if you're one of the countries with a lower number of weapons (i.e. not Russia or USA) - you want to be sure a large portion of them will get through.
    – komodosp
    Nov 14, 2023 at 13:51
  • Why the down votes? This is a perfectly valid answer. MAD only ever applied to the major powers, and even with "pretty good" missile defense systems the major powers still possess the ability to overwhelm the defensive systems. Today's missile defense systems are designed to potentially protect against smaller rouge players.
    – JMS
    Nov 14, 2023 at 15:58
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    @NoDataDumpNoContribution: The radioactive components of nuclear missiles continuously degrade themselves and other components around them. It's far from clear how effective counties like Russia have been in maintaining their arsenal.
    – supercat
    Nov 14, 2023 at 21:15
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    $Hobbamok, So when he says Mutually Assured Destruction MAD still exists you hear him saying it doesn't? When you hear him say no air defense system is 100%, you hear him saying it is? Russia has 5000 nuclear warheads, Even if there was an air defense system which was 90% effective. That's still 500 warheads destroying you. Who knows how effective THAD is. It doesn't take MAD of the table. that's the take away..
    – JMS
    Nov 14, 2023 at 23:58
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    @Peter-ReinstateMonica, This is how I look at it. When guns first were used in battle even a minor flesh wounds were generally fatal. Medicine advanced and today people can survive very serious gun wounds. That doesn't mean anybody is volunteering to get shot or that guns aren't still useful for protecting and attacking countries.
    – JMS
    Nov 15, 2023 at 15:39
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[Even under the wrong assumption that nations had air defenses capable of intercepting even half of incoming ICBM-based warheads]

An intercepted nuclear warhead just becomes a dirty bomb. So if you're intercepting in the later stages of flight (which is what your question refers to) you're actually worsening the radiation falling onto your land because none of it is fissioned.

Sure, your military sites and cities stay intact, but everything is poisoned nevertheless.

This isn't as true for fusion based bombs (which only carry a smallish amount of fissile material), but for fission based bombs this is a major argument. And again: ICBM defense is a myth.

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    Hydrogen bombs carry just as much fissile material, if not more, than regular fission bombs. Most hydrogen bomb designs (that are public knowledge) are boosted fission designs, which depend on one or two fission bombs to jumpstart a fusion reaction. Second point- practical interception technology relies on intercepting missiles in the boost phase or midcourse phase, which for the USA means landing all that dirty bomb material on your adversary's country or over the Pacific Ocean. Third: Nuclear weapons are not all that radioactive until they fission and detonate. Dirty bomb requires reaction.
    – David
    Nov 15, 2023 at 3:20
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Russia possesses on the order of 6000 nuclear weapons.

Of those, under 2000 are ready to launch at a moment's notice.

If 90% of them fail to launch or are intercepted, that is 200 nuclear weapons.

The R-36 MIRV carries 10 warheads and 40 decoys. So those 200 nuclear weapons can be 2000 warheads.

We can reasonably safely assume that this means about 200 urban areas of the USA are destroyed. This includes every urban area in the USA with a population of 138,037 or more. So 100-200 million people incinerated in the direct strike, about half of the population of the USA.

It also involves putting an insane amount of radioactive material into the biosphere. Fallout will poison much of the remaining land. It will also involve an insane amount of combustion and particulate sent into the upper atmosphere. You'll get at least a missing summer if not more (nuclear winter).

A similar amount of devastation will occur in Russia. It is also possible that the war would extend to other nuclear powers - one doctrine is that you know you'll be devastated, so you destroy all rivals not just the one attacking you.

"The bomber will get through" and "The missile will get through" is a standard doctrine -- you can reduce how many, but they'll get through, because offence is cheaper than defence. At the same time, some of your target will survive; so the first wave (described above) will only be part of the problem. So a second strike is expected after this one.

What more, given the raw volume of nuclear weapons this planet has, if they just exploded their weapons without even firing them, we'd still have a global nuclear winter - a human triggered ice age - which would be enough to destroy modern civilization.

If some power wanted to enforce MAD, they could go further. You can engineer a dirty bomb that produces a bunch of radioactive dust and set it off so it spreads globally. Such a doomsday weapon is designed to kill the planet of all human life.

You spread Cobalt-60 with a slightly under 6 year half life. It is intensely radioactive. A strong Cobalt-60 bomb at 10 Sv/hour kills humans in 30 minutes of exposure shortly after the bomb, and it requires 50+ years to survive exposed to the fallout for even a week; nobody builds shelters capable of lasting a century.

So even if someone did build an effective direct missile shield, the attacker who wants to ensure MAD merely has to build Cobalt bombs and detonate them in such a way that fallout lands on the target; the anti-nuclear weapon shield now has to defend not just the land, but the air currents that head over the land.

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    Your argument about MIRVs is wrong. 1674 is the number of warheads so already includes MIRVs with multiple warheads per missile.
    – xyldke
    Nov 16, 2023 at 7:02
  • No, 200 warheads hitting does not imply that "We can reasonably safely assume that this means about 200 urban areas of the USA are destroyed." We have some access to e.g. cold war missile allocation plans of both USSR and USA, and we know for sure that the standard doctrine did not expect to spend most warheads to prioritize urban areas, but rather most of them target missile bases, airfields and other infrastructure. Nor is a single modern warhead (which generally intentionally have smaller yields than earlier ones, probably to fit more of them in MIRV) expected to wipe out a major city.
    – Peteris
    Nov 16, 2023 at 15:11
  • @Peteris I assumed the nuclear weapons where MIRVs. Ie, a single nuclear weapon (missile) has a MIRV; the MIRV doesn't count as 5 weapons just because it has 5 warheads.
    – Yakk
    Nov 16, 2023 at 16:45
  • @Yakk that assumption is not correct, all the global counts of nuclear weapons and the treaties limiting the number of weapons and the various inspections of each other's arsenals due to these treaties count nuclear weapons as nuclear warheads, not the missiles (some of which are nuclear capable but often are equipped with non-nuclear warheads). If you read "country A has X nuclear wapons" that by default implies X warheads, not X*5 warheads; a MIRV missile can deploy multiple nuclear weapons.
    – Peteris
    Nov 16, 2023 at 17:29
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They are not the same missiles. For an ordinary missile, you pay $x and expect y% chance to damage the enemy. If you could build a missile for $2x then you would do that if its chances are more than 2y%, but not if they are less. If you could build a missile for half the cost you would do it if the chance of damage stays better than half. You reach the optimal ratio at a point where many missiles get shot down.

For nuclear missiles, the cost is different. There's the cost for building a nuclear war head which is very high, for building a missile that can carry it over thousands of miles, which is also very high, and for making sure it explodes where it should, which is relatively cheap. So to maximise the damage for the money, you will invest much more into making sure that the missile doesn't just fly 6000 miles but also explodes in the right place. Maybe you can for the same money build 60 missiles of which 50 will be shot down, or 50 missiles where 40 will arrive and explode. So you will do the latter.

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