There have been immense efforts to develop anti-ballistic missile technology. But there have been immense difficulties, both technical and theoretical, in this work.

Nuclear weapons were initially delivered by bomber — long-distance airplanes. So the earliest defensive technologies were anti-bomber; the first was deployed by the US in 1953, only a few years after the first Soviet nuclear test. Among these were surface-to-air missiles that would try to take out said bombers with either conventional or nuclear warheads. When intercontinental ballistic missiles (ICBMs) became a larger threat, this approach carried over towards trying to defeat them as well: shoot down a missile with a missile.

The technical difficulty, here, is that an incoming missile is very fast. A reentry vehicle on a ballistic trajectory might be moving at 17,500 miles per hour. That is many multiplies of the speed of sound. The reentry vehicles of nuclear weapons are physically not that large; the largest are about the size of a car, the smallest (each of those "dunce caps" are one RV) are about the size of a human being. Relative to the size of the sky as viewed from the ground, that is very small. The common metaphor is that it is like "trying to hit a bullet with a bullet" but even that kind of understates the problem.

There were several ABM (anti-ballistic missile) programs from the 1950s onward in the US and USSR. They basically involved using nukes to try to hit nukes, with the hope being that the size of a nuclear detonation might be enough that you only would have to get within a mile or so of the incoming weapon to break it up.

The problem is, even if you have a system like this, it is a) very expensive to deploy — if you need at least one defensive missile for every offensive one, then that's a lot of missiles, and b) for this to even be worth doing, you have to have a high confidence that it is going to work well, because you only need a few nukes to get through your system for it to be a problem. E.g., if they are launching 1000 nukes at you, and you have 1000 interceptors, and an extremely-impressive-to-the-point-of-unlikely 90% success rate, that is still 100 nukes that have hit you. 100 nukes will ruin your day (and country).

Once the US and USSR started working on these kinds of systems, it became apparent to each of them that there was an easy way to defeat them: just make more offensive weapons. So you are jacking up the costs of any defensive system, as well as reducing their use. So instead of 90% leaving you with only 100 nukes going off, 90% might leave you with 1000 nukes going off. Which is nation-destroying.

Even worse, the Soviets and then the US figured out how to do a technology called MIRVing, in which a single missile could have multiple re-entry vehicles on it, targeted at different places. So a missile like the Peacekeeper had 10 different warheads on it, each independently aim-able. Think about what this means for an ABM system: now, instead of needing one defensive missile for each offensive missile, you need ten defensive missiles for each offensive missile. (And arguably more if you think there is a chance that your defensive missiles will miss and you will want to be redundant.) So this just overwhelms the idea of any nation-wide ABM system very quickly.

Oh, and on top of all of this, it is possible to fool ABM systems. They are going to be using radar to spot incoming warheads, right? Well all you need to do to really make things difficult is confuse the radar. Two ways to do this very cheaply is to have your warhead eject "chaff" (think of little strips of metal or foil that will bounce back the radar signals as "snow") or decoys (essentially balloons that look to a radar like a warhead, but aren't). Or as I like to facetiously put it, you have a little birthday party in the sky — confetti and balloons.

So now ABM looks somewhat impractical and very expensive. Worse, and this is the strategic implication, if you develop ABM systems you are creating a dynamic where all sides are highly incentivized to have huge offensive arsenals to counteract them. You are reducing the stability of the entire nuclear deterrence dynamic by both introducing doubt into the possibility that mutual assured destruction might hold, and creating incentives to defeat ABM, some of which might be worse than the initial fear (e.g., deployed missiles in the thousands and thousands).

By the 1960s, even the nuclear strategists in the US were acknowledging that these systems were astoundingly expensive to develop and deploy, and that they did not measurably reduce the Soviet nuclear threat to the United States. By 1972, the US and USSR signed a treaty, the ABM Treaty, that basically put limits of ABM deployments, on the argument that they were actually more dangerous than not. They basically agreed to each deploy only one ABM system total, so that the other side wasn't totally incentivized to build huge, huge arsenals. As an aside, by this point ABM was extremely unpopular amongst the American public, in part because anywhere you deploy an ABM system is now a very high-priority nuclear target for the other side.

In the 1980s, the Reagan administration got very excited about new ABM possibilities that had been dreamed up. The Strategic Defense Initiative involved defenses that would be deployed in outer space itself, not on the ground like previous ABM. So there were studies as to what it would take to put giant lasers in space which could ideally shoot down lots of missiles before they began the descent into the atmosphere that sped them up. The most famous of these schemes was the X-ray laser scheme, in which a nuclear bomb would be exploded in space and its X-rays would be channeled into lasers that might destroy a bunch of nukes all at once. There were many variations of these space defense schemes. They were all immensely expensive and many of them were overhyped (including the X-ray laser, in which evidence of its difficulties was suppressed by the laboratory developing it), and once again it raised the possibility of destabilizing the deterrence. In any event, none of it really worked.

When the Cold War ended, a lot of this work got transformed into a smaller, less-ambitious ABM research program that still is ongoing today in the US. The US left the ABM Treaty in 2002 to pursue these systems. The current program essentially involves multi-tier approaches to trying to shoot down missiles at different parts of their launch (e.g., boost phase, when it is just going up and is very slow, versus terminal descent phase, when it going fast). Some of these systems are currently deployed. But the current ABM systems are a) very limited in how many missiles they could shoot down (the US only has a few dozen defensive interceptors even deployed), so they are mostly about the possibility of a couple ICBMs being shot down, not a full salvo of any major nuclear weapons state, and b) their success rate, even with modern computers and decades of research, is far from the perfection desired. In very coordinated tests (where the defender already knows where the offensive missile is going, and the offensive missile isn't trying to fool the defender, and there is basically only one offensive missile) the system can sometimes shoot down the missile. This is not a realistic situation, but the logic is that it is better to have kind of half-working systems deployed, because a) who knows, you might need them, and b) they will possibly inject enough uncertainty into the mind of, say, North Korea, that they will think twice about trying to use a nuke.

The downside to the current situation is that after the US left the ABM Treaty, it made both Russia and China concerned that the US would eventually develop tech that would possibly negate their ICBMs. And this gets at a broader point: the people who rely on nuclear weapons for their sense of national security are not going to give up and say, "oh, shucks, I guess we're done with nukes." They are just going to find ways to defeat ABM systems, as you would expect. So both Russia and China have been developing weapons systems, to the alarm of the US, that are designed for getting around ABM, whether they are hypersonic reentry vehicles (which are capable of doing non-ballistic trajectories, and so can evade defensive systems very fast), long-range cruise missiles (again, not ballistic), underwater drone torpedos, and expanding their deployed arsenal (in the case of China). All of these change the status quo in tricky ways that arguably introduce a lot of uncertainty into the balance of deterrence, and have caused the US to respond by trying to upgrade its own offensive weapons systems, and so on, in classic arms race style.

So there are a lot of people, myself included, who have been saying for years that ABM is a bad approach to making the world safer from nukes, because it seems to inevitably create responses that make the threat even more dangerous. This is just yet another one of the apparent paradoxes of nuclear weapons: that defensive technologies may be inherently more dangerous than offensive ones, because they cause people to develop even more problematic offensive technologies.

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Hi Xdarkhero,

Great question! I think there are a couple of good reasons why where haven't seen anyone develop a comprehensive defense against nuclear weapons. TL;DR, mutually-assured destruction already provides a sufficient defense, and developing an effective-enough system that works in the long-term is virtually impossible to grantee.

The first is that nuclear weapons development is a continuous game of cat-and-mouse development. Just as one country might develop ways of defeating another's nuclear weapons, so that country will work on getting around those defeats. Consequently, while you might be able to hinder the nuclear capabilities of an enemy nation temporarily, any one method is unlikely to be successful on a sustained basis, and achieving even that can require significantly more investment than getting round it does.

This has the side-effect of discouraging too much investment into countering nuclear capabilities, as that investment will often soon prove worthless if the opposing side changes their tactics, rendering resources that could have been used elsewhere wasted.

To take one example of this, at the start of the cold war, most nations planned to deliver nuclear weapons as bombs dropped by jet aircraft at very high altitude to fly above enemy air defenses and avoid the blast of a nuclear weapon, allowing for more destructive bombs. To this end, the RAF ordered a fleet of such aircraft and kitted them out for these sorts of operations against the soviet union. In response, the USSR invested significant resources in developing increasingly capable anti-air missiles like the SA-2 that these aircraft could not fly over. However, when the RAF realised their established strategy was no longer viable, they were able to negate the system simply by re-training their bomber crews to fly very low-level sorties, which cost them no more than slightly-more-frequent maintenance inspections, yet allowed them to maintain a nuclear deterrent against the USSR.

Such cat-and-mouse development is not uncommon within the military-industrial complex, obviously, but Nuclear warfare has a few unique characteristics that make it particularaly fruitless to invest against.

First, the scale of the threat posed by nuclear weapons means that, for most people, nothing less than a 100% effective defense would ever be considered good enough to trust in. If there was even a 1% chance that one nuclear weapon could slip through your net, that could still cause millions of casualties to your side. Consequently, a nuclear weapon defense that is merely 99% effective is as good as useless to most military planners, because they would still be forced to treat any other nuclear-armed nation as if they could successfully strike you in a war, just as if you had no defense. The snag is it's almost impossible to guarantee your defense will be 100% effective, let alone if it will still be by the time it's fully operational, let alone for exactly how long after it will remain 100% effective against the enemy's attempts to circumvent it. Yet you still have to commit billions of pounds in funding this program right at the start on the off chance that you might be 100% sucessful for a meaningful amount of time, but with no way of guaranteeing that. For almost any government, that is nowhere near worth it when those resources could be plowed into other parts of their armed forces.

The second thing to consider is that the only countries that possess the expertise and means to have any hope of developing such a system are the richest and most technologically adept countries on the planet. However, those countries already have an effective defense against nuclear attack: their own nuclear weapons. Whereas an anti-nuclear defense needs to be effective 100% of the time, one's own nuclear arsenal only needs to have a chance of being successful once to be effective deterrent to an enemy attack. Consequently, retaliatory nuclear weapons can be upgraded less frequently and expensively than a comprehensive nuclear defense umbrella, while still providing a more credible deterrent for longer. Consequently, Britain is able to still use nuclear weapons almost 50 years old to provide a credible deterrence, despite them being some of the oldest weapons in the Royal Navy's arsenal.

Thirdly, taking steps to negate the threat of mutually-assured destruction can be undesirable, as it can encourage the use of nuclear weapons if one country feels they might be able to use them without facing significant retaliation. This can make the possibility of nuclear war, with all its attendant death and destruction, more likely, and could threaten to disrupt the balance of power between major military nations that has helped ensure they've remained mostly at peace since 1945. While every country would probably be more than happy for one country to develop such a system if they could guarantee it was them, that has not been the case for most of the post-war period, and with the stakes so high, even the slightest risk you'd be the one under the nuclear hammer is too high. Consequently, for most of the post-war era, it has been in most countries' interests to try to prevent such a race occurring, just in case they were second to the punch. This has created an international consensus to maintain the threat of mutually assured destruction for everyone, leading to arms-reduction treaties like the Anti-Ballistic Missile Treaty that formally limited the capabilities and extent on the US and USSR's anti-nuclear defenses.

Consequently, those states with the means to develop an comprehensive-enough defense from nuclear attack largely haven't bothered to, because they have more cost-effective and capable means at their disposal through their own nuclear arsenals (or hosted weapons in the case of Germany and Italy), and because the long-term complete success of any such system is too doubtful to make the monumental investment necessary to achieve it worthwhile.

Hope this helps :)

Have a lovely day

ps As a bonus, however, there are certain circumstances where nations have started the development of such programs, the most famous of these being Reagan's 'Star Wars' Strategic Defense Initiative, which proposed the development of a comprehensive set of space-based anti-missile lasers that could swat any Soviet nuke out of the sky before it had a chance to harm a single american hair. In these cases however, the wastefulness and short-term nature of such programs are its benefits, not its fatal drawbacks. Reagan sought to leverage the economic superiority of the USA to put additional pressure on the Soviet Union by backing the most resource-intensive and wasteful government program possible that they would be forced to respond to: a comprehensive ballistic missile defense. To maintain the effectiveness of their deterrent, the USSR would have to either develop a similar system of their own, or re-structure their entire nuclear arsenal to avoid the system, neither of which they could afford by the 1980s, unlike the US.

Some great explanations so far. I’ll make mine brief.

Since the beginning of the Cold War and the proliferation of nuclear arms, the philosophy underpinning global peace has been mutually assured destruction (MAD). In short, the idea of MAD is that if two separate civilizations, A and B, both possess nuclear warheads in large enough quantities and both have many mechanisms by which to deliver those warheads (namely, by air, land, and sea - the nuclear triad), then it stands to reason that in the event of armed conflict, both civilizations A and B possess the means to bring about the complete and total destruction of the other. Seeing as the rational person does not want to bring about the end of their civilization, this idea acts as a deterrent to war, and more specifically, as a deterrent for nuclear weapons use and proliferation.

Now, one more idea to bring in. The idea of “second strike capability”. Nuclear weapons make big explosions. Military leaders started to wonder, hey, what if the other civilization launched a nuclear attack that was so large, so broad, so sudden and so swift, that it completely destroyed our civilization before we ever had the chance to respond? In that event, our “second strike capability” is zero, and the philosophy of mutually assured destruction fails. So how do we respond? By making certain that neither side possesses the technology to completely disarm and destroy all nuclear missives from the other civilization.

So, in short. Both civilizations A and B have nuclear weapons. And both civilizations have technology to disarm incoming nuclear missiles. But, as the defense technologies of one side get better, the other civilization makes smarter missiles, which leads to smarter defenses, which leads to smarter missiles… in short, a vicious cycle that has been repeating for decades now.

I have to jump on this with a question, i know it’s very far down.. but realistically, IF we have the technology to stop a nuclear weapon and IF we could stop being nuked.. would it be information that is made public?

I know that seems like a silly question, and I’m not onto jumping into conspiracies, nor am I educated - my knowledge is as far as fallout 3 and nukacola

But would it be something that would be known? As surely it would be showing your cards and causes more reason to call a bluff..

But another question, and I suppose one to ease the overwhelming new anxieties I have over this… would it actually happen.. could we be nuked? I just don’t understand why - but then again I don’t understand war or violence..

I’m sorry again for jumping into something with no education and asking some quite stupid questions.. but I have to admit, there is a fear that’s been awoken from this as I’ve actually always liked russia (until recently because of the threats)