|Fig. 1: Minuteman III in silo, 1989. (Source: Wikimedia Commons)|
Since its development during the Cold War (1947-1991), the United States' "nuclear triad" has retained the important role of bolstering US national security. The "triad" refers to the three strategic nuclear delivery vehicles: strategic manned-bombers, land-based intercontinental ballistic missiles (ICBMs), and submarine-launched ballistic missiles (SLBMs) - each of which offers a unique function that broadens US nuclear capabilities. This multi-faceted approach effectively diversifies the risk involved with a potential first-strike nuclear attack - and increases the ability of an offensive second-strike attack. While the government remains confident in the reliability of the triad as a nuclear deterrent, new dialogue has been established regarding the relevance and feasibility of maintaining specific legs of the triad in the 21st century.
The first leg of the triad, the strategic manned-bomber, delivered the first atomic bombs during World War II. Over the decades, these American-made bombers (B-29, B-36, B-52, B-2) have exhibited a combination of speed, distance, and weight bearing capacity that has outperformed alternative foreign models. Bombers can be quickly deployed and conversely, quickly recalled in response to last minute decisions. In addition, bombers have the ability to carry both nuclear cruise missiles and more conventional weapons - allowing the Air Force to utilize them during non-nuclear missions. Because of this flexibility, strategic bombers have been an essential part of the triad and can function as both a first and second strike attacker. 
The implementation of New START Treaty (an adaptation of the Strategic Arms Reduction Treaty) will reduce the US fleet of bombers to 60 aircraft (reducing the current size of the B-2 and B-52 programs from 20 and 54 to 19 and 41, respectively).  The B-52, America's most produced strategic bomber, is more than 60-years-old. However, because the strategic bomber is viewed as a particularly important leg of the triad, instead of retiring it with age, the fleet will be modernized. The Air Force is currently in the process of developing a new bomber for longer-range strike missions. 
ICBMs are silo-housed, long-range missiles that are located in controlled (or allied) territory, see Fig. 1. They are the most numerous delivery vehicles in the US triad - consisting of 414 land-based Minutemen III ICBMs. Due to their geographical dispersion in the US and nuclear blast proof silos, ICBMs are well protected against first strike attacks (although some claim their stationary nature makes them more vulnerable). ICBMs are also noted for their low relative cost per use compared to other legs of the triad and the promptness in which they can be launched. 
The New START Treaty will reduce the current 414 ICBMs to 400 deployed missiles.  While plans are in store to modernize these missiles (specifically, the Ground Based Strategic Deterrent), many argue that the Air Force should abolish the ICBM force completely. Former Secretary of Defense William Perry believes that spending on ICBM modernization would be wasteful since strategic bombers and SLBMs provide the most effective and sustainable means of nuclear deterrence. Additionally, due to the difficulty of launching ICBMs over Russia without destabilization, ICBMs suffer from what is called "targeting inflexibility" which greatly limits their use - and ultimately, strategic value. 
The SLBM is often considered the most critical leg of the triad. The rationale comes from the fact that submarines are very difficult to track and destroy - making them almost invulnerable to first strike attacks and increasing their value as a second strike attacker. Initially, due to the nature of a floating submarine, the SLBM was the least accurate delivery vehicle and was limited by the difficulty of obtaining accurate targeting data. Now, with improved guidance and communications, SLBMs' accuracy is almost at par with that of an ICBM. Despite their effectiveness, SLBMs represent the smallest component of the triad with only 14 deployed Trident submarines - this is largely a factor of their expense.  While the majority of SLBMs are not as old as the B-52 bomber, there is no question that they are in need of modernization. Recognizing this, the Navy has begun to design a new Columbia-class submarine that will serve to replace the existing Ohio-class fleet. 
While each leg of the triad serves to strengthen US national security, each comes with a value relative to its respective costs and benefits. All three legs are aging and require vast amounts of capital to maintain - it is estimated that over the next 30 years, the US will spend over $1 trillion to modernize the nuclear triad.  However, as a result of spending cuts and fundamental questions regarding the effectiveness of US nuclear deterrence against modern day threats (terrorists, rogue regimes, etc.), many have started to question the relevance of the nuclear triad. It is still to be determined how the size and structure of the nuclear triad will change in the coming years.
© Jack Chabolla. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 A. F. Woolf, "U.S. Strategic Nuclear Forces: Background, Developments, and Issues," Congressional Research Service, RL33640, February 2017.
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 J. B. Wolfsthal, "Rightsizing the U.S. Nuclear Arsenal," Arms Control Today, 43, No. 7 (September 2013).