Three Legs, One Purpose, and Why America Keeps a Nuclear Triad

Your grandmother told you not to put all your eggs in one basket, and she was right, and she did not have a Ph.D. in strategic studies either. The most expensive and least discussed application of that advice is the American nuclear triad. We keep our deterrent in three baskets: land, sea and air. Each basket has a different weakness, and each one covers for the others. The whole thing is designed to deliver one message to anyone who is thinking about it: you cannot win this, so do not try.

It is not a glamorous subject. Nobody has ever won an election by promising to modernize a submarine-launched missile. But it is, quite literally, the thing that stands behind every other thing the government does. If you want to understand why a serious country spends serious money on weapons it hopes never to use, you need to understand the triad. So let's take it apart, leg by leg, and then ask the hard question, which is whether three is too many.

The one-sentence purpose

The purpose of the triad is to make sure that no adversary can ever calculate that a surprise first strike would eliminate America's ability to strike back. That is all. The weapons are not meant to fight a war. They exist so that nobody starts one. A nuclear force that survives an attack is a force that deters the attack. A nuclear force that might not survive is an invitation.

Note what this implies. The key quality is not size and not even raw power. It is survivability. A smaller force that is certain to survive is a better deterrent than a bigger force that could be caught on the ground. Everything about the triad follows from that.

Leg one: the land-based missiles

The first leg is the intercontinental ballistic missile, the ICBM. These are missiles housed in underground silos, spread across a belt of sparsely populated states in the northern plains and the Rocky Mountain region. The missile in service for decades is the Minuteman III, which first entered service in 1970. A replacement program, known as Sentinel, is meant to succeed it.

What does this leg do well? Several things.

• Speed and readiness. An ICBM can reach its target in about half an hour. The missiles sit on alert, controlled by launch crews who spend their shifts underground, and the command system is built to be dependable.
• Accuracy and control. The force is tightly tied to the national command system, so the President can reach it.
• A dispersed target set. The silos are scattered over a very large area. To destroy them, an attacker must hit each one, and he must use enough warheads on each to be sure.

That last point is what military planners call the "sponge." The land-based force soaks up an enormous number of enemy warheads in a first strike. An attacker who wants to take it out has to spend a large part of his own arsenal on American soil in the process, and he has to start a nuclear war with an attack on the American heartland that can leave no doubt about what has happened. That is a big thing for any leader to do on the basis of a calculation that the survivors will not retaliate.

What does it do poorly? The silos are fixed and their locations are known. In theory, the leg could be destroyed in the first minutes of an all-out strike, and that creates pressure to launch the missiles if a warning says they are under attack. Critics say that is a recipe for catastrophe by mistake. Hold that thought, because it is the best argument against this leg, and we will come back to it.

Leg two: the submarines

The second leg is the ballistic missile submarine, which the Navy runs. The class in service for decades is the Ohio class, armed with the Trident II missile, and a new class, the Columbia, is being built to replace it. The first of these boats on a deterrent patrol was the USS George Washington, which put to sea with Polaris missiles in late 1960, and the system has been refined ever since.

The strength of this leg is simple: it hides. A submarine at sea is very hard to find. It can stay submerged for long patrols, and its location is among the best-kept secrets in the American government. An adversary who wanted to preempt it would need to locate and destroy every boat on patrol at the same moment, and nobody has been able to count on that. This is the "ride out" leg of the triad. Because the submarines are survivable, a President does not have to decide in a panic. He has time to think, and that time is a form of safety for everyone, including our enemies, because it makes an accidental war less likely.

Its weaknesses are mostly practical. Communication with a submerged boat is more complex than with a missile crew in a silo. A submarine needs to be maintained, which takes time in port, and the fleet is limited. Technology changes too: if some future breakthrough in detection makes the ocean transparent, that leg becomes vulnerable. Nobody can prove it will not. That uncertainty is exactly why you do not want to depend on a single basket.

Leg three: the bombers

The third leg is the long-range bomber. The B-52, which has been in service since the mid-1950s, the B-2 stealth bomber, and the B-21 now in development carry gravity bombs and cruise missiles. This is the oldest leg, dating from the days of the Strategic Air Command in the 1950s, and it is the only one of the three that can be recalled.

That quality is more valuable than it sounds. A missile, once launched, cannot be called back. A bomber can turn around. In a crisis, a President can order bombers into the air as a visible signal, a demonstration of resolve that does not commit the country to anything irreversible. Bombers can be dispersed to different airfields and put on alert. They can carry conventional weapons as well, which makes them useful every day. They also show the flag, which is a form of deterrence, and they can be seen.

Their weakness is time. A bomber takes hours to reach a distant target, not minutes. The aircraft must penetrate air defenses, which is why stealth and standoff weapons matter. And a bomber sitting on a runway is a target, which is why the base alert posture counts.

The attacker's dilemma, with arithmetic

Now put the three together and look at the problem from the point of view of the man who would like to strike first. He has three separate jobs, and the solution to one makes the others harder.

To take out the missiles, he has to launch a massive and immediate strike on the plains. To take out the bombers before they get off the ground, he needs warheads on the airfields within minutes, which means launching from submarines parked near our coast, and that exposes his own boats to detection. To take out our submarines, he has to find them, which is an intelligence and anti-submarine problem of a completely different kind. No single weapon, and no single plan, solves all three at once.

Here is a labeled hypothetical to show why that matters. These are not real probabilities, and no one knows the real ones. Suppose an attacker's planners were 90 percent confident of destroying each of the three legs, and suppose the three efforts were independent. The chance of destroying all three is 0.9 × 0.9 × 0.9 = 0.729, about 73 percent. That leaves more than a one-in-four chance that some part of the American force survives and retaliates. If his confidence in each is 70 percent, the result is 0.7 × 0.7 × 0.7 = 0.343, about 34 percent. And in the real world, no planner can honestly be 90 percent sure about finding submarines at sea. The point of the example is not the numbers. It is that stacking three hard problems produces a plan nobody would stake a country on.

That is what deterrence needs: uncertainty in the mind of the one who would attack. If you want the concept in its general form, Why Deterrence Is a Promise You Have to Be Willing to Keep explains why belief is the whole game.

How the triad got built

The triad was not designed on a whiteboard in one afternoon. It grew in the 1950s and early 1960s. The bombers came first. The submarine-launched missile arrived with Polaris at the end of the 1950s. The land-based Minuteman missile entered the force in the early 1960s. By the 1960s all three legs were in place, and the idea that each should back up the others had become the settled logic of American strategy.

The modernization fights are an old story too. President Carter canceled the B-1 bomber in 1977. President Reagan revived it, as the B-1B, as part of the strategic modernization program he announced on October 2, 1981. The MX missile, the successor to the older land missiles, was stuck for years in a debate over how to base it so that it would be survivable, and a commission headed by Brent Scowcroft in 1983 helped break the logjam by recommending that it go in existing silos while the country worked on a smaller missile. The point is this: each time a leg of the triad has come due for replacement, there have been voices that said we could skip it. And each time the country has had to choose, in a messy political fight, to fund the insurance.

The strongest case against the triad

Here is the objection, stated as its smartest advocates would state it. Some serious people, including former Defense Secretary William Perry, have argued that the land-based leg is not needed, and that the country could rely on submarines and bombers. Their reasoning is that ICBMs are the leg most likely to cause a catastrophe by mistake, since the fixed silos create pressure to launch on warning, and the missiles' only real function is to absorb enemy warheads. They also argue that the money could be spent on other, more usable forces.

It is a fair argument, and I'd respond in four points.

1. The "sponge" is not a bug. It is the point. A leg that forces the enemy to spend a very large share of his arsenal on American soil is a leg that makes a first strike unattractive.
2. Dropping to two legs makes the attacker's problem easier. With two, he has two jobs instead of three, and a single technological breakthrough against submarines could leave the bombers as the only backup.
3. The "launch on warning" fear is real, but it is managed by the command system, by the human decision points in it, and by the existence of the submarine leg, which means the President is never forced to use the silo missiles under pressure.
4. Retiring a leg is not something you can undo. Once the production lines, the crews and the sites are gone, they will not come back in an emergency. The decision to give up insurance should not be made in a budget cycle.

I'd add one honest conservative caveat. A thing being necessary does not make its price tag sacred. The same discipline we apply to every other program applies here: audits, schedules, penalties for contractors that miss them. For a primer on how weapons get bought, see From Requirement to Runway, How the Pentagon Buys a Weapon, and for how to read the cost numbers you will be shown, A Worksheet for Reading Any Defense Budget Number Without Being Fooled.

Why modernization lands all at once

The reason this argument returns every few years is a plain accident of history. The Cold War buildup put most of these systems into service within a few decades of each other. So they age out of service at about the same time, and the bills come due together. That is the so-called "bow wave," and it is a political problem more than a technical one. Congress delays, because the bill is large and the threat is not on the front page this week. Delay raises the bill, because you have to repair aging equipment, and it narrows the options, because the old systems keep running past their design life. The cheapest time to replace a deterrent is the moment before anyone thinks it is needed.

This is where fiscal conservatives and defense hawks should find common ground, rather than fighting. The worst outcome is the one we get when we starve a program for years and then pay a panic price. It is the same pattern described in Peace Through Strength Is Older Than Reagan and It Still Works. And for those who argue about how large the American commitment should be, the factional debate is laid out in Hawks, Restrainers, Realists and Nationalists on the Right, Compared.

The bottom line

• The triad's purpose is survivability: land, sea and air legs so that no first strike can be sure of ending America's ability to answer.
• Each leg covers for the others. Silos soak up warheads, submarines hide, and bombers can be recalled and shown as a signal.
• Stacking three hard problems turns a first strike from a plan into a gamble, and that is the whole idea of deterrence.
• Modernization comes due all at once because the Cold War built most of it all at once. Delay does not save money. It raises the bill.
• Ask your representative three questions: what does each leg do, what happens to the deterrent if we skip a generation, and how will we hold the contractors to the schedule?