by Kingston Reif and Travis Sharp
Updated by Usha Sahay
Since 1959, the United States has maintained a “triad” of nuclear weapons delivery systems, which consists of nuclear-armed bombers, land-based missiles, and submarine-launched missiles. Traditionally, it has been argued that the triad, with its three distinct delivery platforms, offers the U.S. a diverse set of options to deter adversaries, assure allies, and defeat adversaries if deterrence fails. The 2010 Nuclear Posture Review conducted by the Obama administration stated: “Retaining all three Triad legs will best maintain strategic stability at reasonable cost, while hedging against potential technical problems or vulnerabilities.” It went on to note that “each leg of the triad has advantages that warrant retaining all three legs.” However, in recent months, a confluence of strategic, political, and economic factors have prompted further scrutiny of the U.S.’ continued reliance on a nuclear triad.
First, news reports in early 2013 suggested that the President could agree to changes to high-level nuclear weapons policy guidance that could reduce U.S. deployed strategic warheads to between 1,000 and 1,100. This lower level, whether implemented bilaterally with Russia as preferred by the Obama administration or unilaterally, could also be accompanied by reductions in delivery systems. In addition, the prospect of $1 trillion in defense spending reductions over the next decade due to the Budget Control Act, have led some to question the affordability of replacing the existing triad. In July 2011, then-Vice Chairman of the Joint Chiefs of Staff Gen. James Cartwright stated that “The challenge here is that we have to recapitalize all three [triad] legs and we don’t have the money to do it.” In the event further reductions in nuclear forces or fiscal pressures begin to raise questions about the feasibility of maintaining the existing force structure, greater scrutiny will be placed on the arguments traditionally made about the relative strengths and weaknesses of each leg of the nuclear triad. 
SUBMARINE-LAUNCHED BALLISTIC MISSILES
Note: All U.S. nuclear forces data is from Robert Norris and Hans Kristensen, “Nuclear Notebook: U.S. Nuclear Forces, 2013,” Bulletin of the Atomic Scientists. All Russian nuclear forces data is from Norris and Kristensen, “Nuclear Notebook: Russian Nuclear Forces, 2013,” Bulletin of the Atomic Scientists.
• Because of its dispersion, mobility, and concealment, an SLBM force is effectively invulnerable while at sea.
• The invulnerability of SLBMs not only offers the assured retaliatory capability that is the bedrock of deterrence, but also ensures that the president will not face the “use them or lose them” pressure presented by ICBMs.
• A 1993 Government Accountability Office study concluded “…that submerged [nuclear-armed submarines] are even less detectable than is generally understood, and that there appear to be no current or long-term technologies that would change this.” Non-acoustic detection technologies, such as green-blue lasers and magnetic anomaly detection,continue to advance but have yet to yield any breakthroughs.
• There are only a limited number of nuclear-armed submarines on patrol at any given time. If an adversary were to develop a way to locate these submarines, a disarming first strike might wipe out a SLBM force by concentrating on relatively few aimpoints.
• An attack on a SLBM force might be difficult to attribute. Submarines (and bombers) could theoretically be destroyed through purposeful attrition by a nation seeking to gain nuclear dominance. However, submarines could also be destroyed through operational accidents or in an attack by a nation not seeking to gain nuclear dominance. These scenarios would create uncertainty about whether the submarine force was under deliberate attack and/or whether a nuclear first strike was imminent. Such uncertainty could lead to mistaken escalation and/or retaliation.
WHY PRUNE SLBMs?
Nuclear-armed submarines remain the most survivable leg of the nuclear triad. While further reductions might entail reductions to all three legs of the triad or discussions about eliminating one leg, a safe and secure SLBM force seems destined to remain the centerpiece of deterrence – especially in the United States – for years to come.
*This number is the sum total of “primary mission” aircraft, which are designated for military missions, as well as aircraft in the inventory which are used for other purposes such as training, testing, and backup.
• Unlike land- or sea-based missiles, bombers can be recalled after launch. In a crisis situation, this would enable the cancellation of a bomber strike after it had been ordered if new information emerged and/or the president changed his or her mind.
• Bombers can be dispersed from their bases quickly in order to survive a nuclear first strike. Thus, the president would not feel the pressure to “use or lose” bombers during a crisis. Such pressure might not exist with immobile land-based missiles which are less vulnerable.
• U.S. bombers carry the nuclear weapons of lowest yield, which means that nuclear-capable bombers could potentially provide the president with less devastating options when launching a nuclear attack.
• Because bombers are recallable, “scrambling” them toward a potential target is a highly visible way of demonstrating resolve to adversaries and allies without actually launching a nuclear weapon. Such a demonstration of resolve might deter a potential adversary and thus prevent war. Land- and sea-based missiles offer no analogous capability. For instance, in March 2013, the U.S. conducted military exercises with B-2 bombers over the Korean Peninsula to signal a continued commitment to protect South Korea from potential North Korean aggression. In this way, bombers are the most advantageous leg of the triad when it comes to the United States’ obligations to provide extended deterrence to allies.
• Bombers offer an alternative to missiles in assuring strategic penetration. If a nuclear force were entirely deployed on missiles, that force might be neutralized – thus negating assured destruction and deterrence – by an adversary’s deployment of a workable missile defense system.
• U.S. strategic bombers today are located at only three continental bases in Louisiana, Missouri, and North Dakota. The bomber force could therefore be significantly damaged by a surprise attack against only three targets.
• Because bombers can carry both nuclear and conventional payloads, during a crisis an adversary might suspect that a conventionally-armed bomber was actually carrying nuclear weapons, thereby encouraging an attack against the bomber or a preemptive nuclear strike.
• Instead of demonstrating resolve and deterring an adversary, scrambling bombers might actually exacerbate tension by rousing suspicion of an impending nuclear strike, thereby prompting further escalation.
• Though assured strategic penetration is vital to deterrence, the odds are vanishingly small that an adversary could ever deploy a missile defense system robust enough to stop 100 percent of incoming long-range missiles. The alternative penetration option offered by bombers may not be a unique advantage.
WHY PRUNE BOMBERS?
U.S. nuclear submarines and Russian land-based missiles are the most valued legs of each nation’s respective nuclear triad, making joint elimination of either leg unlikely in the near-term. In contrast, deployed Russian and U.S. bombers are outdated, and U.S. bombers are increasingly tasked with conventional missions, suggesting that reductions of nuclear-armed bombers may be a potentially palatable option for both nations.
LAND-BASED INTERCONTINENTAL BALLISTIC MISSILES (ICBMs)
• ICBMs are the most numerous delivery vehicles in both the U.S. and Russian triads. Moreover, they are geographically dispersed in hardened, underground silos.
• ICBMs are also cited by many as the promptest leg of the triad, offering the US the ability to launch a nuclear attack more quickly than the other two legs.
• As opposed to bombers and sea-based missiles, which are bundled onto relatively few aimpoints and may be vulnerable to attack, the size, protectedness, and dispersion of ICBM forces makes them virtually impossible to destroy short of an all-out nuclear attack. In this way, ICBMs are “warhead sinks,” theoretically less vulnerable and therefore a stronger deterrent to attack by adversaries.
• Because an ICBM force can only be crippled through a large and unmistakable nuclear attack, ICBMs provide greater clarity about when a country is under attack and who the attacker is.
• According to the Congressional Research Service, the cost per delivery vehicle for U.S. ICBMs is approximately one-quarter what it is for bombers and submarines.
• Despite large numbers and dispersion, ICBMs are still theoretically vulnerable to a disarming first strike from Russia because they are immobile and thus more easily targetable.
• ICBMs contribute less to nuclear deterrence – extended or otherwise – than bombers and submarines do, because they cannot be forward-deployed in a particular location as a signal of U.S. resolve.
• ICBMs may not, in fact, be significantly prompter than SLBMs, which, if true, would neutralize a key advantage of ICBMs.
• Because ICBMs are theoretically still vulnerable, the president might feel pressure to “use them or lose them” in the event of a crisis. This pressure might lead to a rapid decision to launch ICBMs based only on receiving warning of an impending attack.
• ICBMs are also poorly suited for launching a nuclear attack against adversaries other than Russia. Targeting, say, North Korea or Iran would require ICBMs to fly over Russian airspace, which risks triggering Russia’s early-warning systems and inadvertently causing a nuclear conflict.
WHY PRUNE ICBMs?
Supporters point out that ICBMs are the least expensive leg of the triad to maintain. However, the difficulty of launching ICBMs without overflying Russia – a problem known as ‘targeting inflexibility’ – makes them a relatively less attractive option for a nuclear strike. Similarly, the inability to forward-deploy ICBMs minimizes their ability to contribute to nuclear deterrence, suggesting that reductions to the ICBM force would not significantly hamper the US nuclear deterrent. Note that the Pentagon has not yet released its preferred force structure for the nuclear triad under the New START Treaty, but is expected to do so by the end of 2013. All New START reduction projections are based on currently available information, compiled by Amy Woolf, “The New START Treaty: Central Limits and Key Provisions,”Congressional Research Service: April 10, 2013.