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The Rise and Fall of the Pentomic Army

Ronald Reagan and Mikhail Gorbachev failed to reach an agreement on the elimination of nuclear weapons while in power, but both men agreed in 1985 “that a nuclear war cannot be won and must never be fought.” These words were not always widely accepted and, for decades, the United States and the Soviet Union rehearsed fighting and winning a nuclear war. In one such scenario from early in the Cold War, U.S. Army units would be asked to use nuclear weapons on the battlefield to offset perceived conventional disadvantages and win a nuclear war before it escalated to the use of long-range missiles.

Developing this doctrine was left up to the U.S. Army, but the service was left without a clear role for nuclear weapons at the dawn of the atomic age. During the 1950s, the Army developed methods to fight a “limited atomic war” that was to occur without an exchange of long-range nuclear missiles at each other’s populations. This time in Army history is referred to as the “Pentomic Era,” in reference to five-sided pentagonal division structure implemented during this period known as the Pentomic Division, which was in effect from 1956 to 1962.

The lessons from the Pentomic Era are many, but Army leaders quickly understood that a reliance on nuclear weapons was disadvantageous — a truism that has held true ever since. The Pentomic Division was a lighter division concept that included five subordinate battle groups of five companies each as opposed to the three-sided structure of World War II and Korea. However, leaders quickly realized this was too “light” for conventional combat and also lacked the proper command and control to effectively use nuclear weapons when dispersed. After four years of experiments, the Pentomic Division was disbanded.

As the world grapples with nuclear brinksmanship once again following Russia’s invasion of Ukraine and thinly veiled statements to use nuclear weapons to stem losses on the battlefield, the lessons from the Pentomic Division and this time in American history could prove salient yet again. Despite the division structure’s failures, the army developed doctrine and techniques to fight on a limited atomic battlefield designed to blunt a Soviet invasion of Germany. These ideas are important for any force pondering a war which could include the use of tactical nuclear weapons to stave off defeat against a conventionally armed foe. The U.S. Army thought about and planned to fight in an atomic environment, building offensive and defensive doctrinal ideas to stave off a Soviet attack. However, the idea was stillborn, and the reasons for the abandonment of the Pentomic concept could offer lessons as policymakers grapple with concerns about Russian nuclear use in Ukraine. 

A Catch-22 of Cataclysmic Proportions

Low-yield weapons are often referred to as “tactical” nuclear weapons. That phrase is misleading terminology, implying that tactical atomic warfare is somehow devoid of strategic significance if kept to military targets on the battlefield. It is hard to imagine any nuclear weapons not having strategic effects. Whether referred to as “tactical,” “non-strategic,” “battlefield,” or “low-yield,” their use by Russian forces in Ukraine would have a massive impact on international relations, either by eliciting a nuclear response from the United States and NATO or an escalated conventional war that draws in outside powers. NATO and the United States would, in all likelihood, agree that a response was warranted to any Russian military use, which could entail a conventional strike on Russian forces. Such action could then spur a Russian response outside of Ukraine, creating the catalyst for escalation to large-scale nuclear use — a catch-22 of cataclysmic proportions.

The American concept of tactical atomic weapons doctrine was borne of the Army’s effort to remain relevant during the Eisenhower era. Upon reaching the White House, President Dwight D. Eisenhower ushered in new national security policies that sought to get the most “bang for the buck,” and thus placed a premium on the aerial delivery of atomic munitions. This “New Look” at national security in the “air-atomic era” starved the Army of its budget and created the conditions in which the Army visualized a limited atomic land war against the Soviet Union. This was to be a war that did not include atomic strikes on cities but rather tactical exchanges of nuclear weapons between land forces. 

At the dawn of the nuclear era, American military strategists sought to develop doctrine to fight and win a nuclear war because Army leaders believed that future conflicts would feature atomic weapons. To fight and win, Army leaders argued that mobility and dispersion would be critical for survival and battle field success. The Pentomic Division would be mobile and be capable of dispersing, increasing the survivability of its forces. The Army was also developing guided missiles and rocket artillery capable of firing conventional and nuclear munitions. The intent was to provide long-range, all-weather supporting fires to highly mobile ground units in support of army doctrine.

This doctrine was based, in part, on the American experience in the Korean War. During this conflict, Chinese forces used overwhelming numbers in what was described as “human-wave attacks.” American leaders feared that the Soviet Union would follow a similar doctrine and, given its numerical superiority, quickly over run defensive lines in Europe. The 1954 edition of Field Manual 100-5: Field Service Regulations, Operations instructed commanders to use atomic munitions as “additional firepower of larger magnitude to complement other available fire support for maneuvering forces.” One way to offset numerical superiority was with technology and atomic weapons, but how to best use nuclear weapons on the battlefield required a change of division structure and doctrine. 

The Pentomic Structure

After coming up with various ideas for dispersed division structures, the Army used Exercise Sage Brush in the fall of 1955 to test different nuclear war fighting concepts. Observers noted the increasing number of subordinate elements assigned to commanders at all echelons as planners tried to determine the maximum number commensurate with available means of control. This test forced the Army to re-evaluate its triangular infantry division structure, which had been in place since 1939. This structure featured three regimental combat teams with three battalions each, a division artillery brigade with three 105mm howitzer batteries and one 155mm battery, and various support elements. The triangular structure was designed to allow commanders to commit two regiments (and each regiment to commit two battalions) to the front, while keeping the third in reserve. The design was perfectly suitable for a conventional conflict because it operated on the principle of mass and provided sufficient firepower, maneuverability, and flexibility within its three regimental combat teams. However, on a nuclear battlefield, this approach was potentially calamitous. The defending force could, the theory went, use atomic weapons to blunt an advance and to attack the battalion in reserve.

The Army settled on a pentagonal structure that reflected what then-Army Chief of Staff Maxwell D. Taylor had experienced while commanding the 101st Airborne Division in World War II. The Army determined that it needed fully air-transportable units to provide quick response capability and that its divisions needed to include all arms and essential supporting services. The Army needed its combat units light enough for its equipment to fit in Air Force aircraft capable of rapidly moving to forward American divisions in Europe or Northeast Asia and reinforce units in the area in the event of a major war. 

The Pentomic division was intended to be fully air-transportable and based on five self-sufficient Battle Groups, each with five companies, a headquarters company, and a mortar battery. This design eliminated the Regimental or Combat Command intermediate headquarters and increased a division commander’s span of control. Division artillery was reduced to five 105mm artillery batteries and a “dual-capable” Honest John rocket artillery battery, which could carry either a conventional or nuclear payload. The Army integrated nuclear armament at the corps and field levels to provide depth and flexibility using missile groups and battalions. The Pentomic reorganization also expanded division reconnaissance companies to battalion size to satisfy the increased target acquisition requirement for nuclear weapons. 

Fighting in the atomic age, American planners assumed, required adjusting current doctrine and unit organizational structures to deemphasize massed forces to present a less enticing target for Soviet nuclear weapons — while still being able to mass into larger formations for offensive action. The five battle group structure was to fight alone in all directions, if necessary, creating islands that forced the enemy to present themselves as targets for the division’s internal tactical nuclear weapons.

By implementing the new divisional structure, the Army hoped that they would force a reevaluation of tactics and doctrine. However, this doctrine was never actually codified in an updated Army field manual. Instead, much of American atomic doctrine comes from professional development journals and books written by senior officers. As a result, few officers at the Battle Group level and below trained in nuclear tactics. Personnel within the battle groups were instead trained primarily for conventional war with the expectation that they would adjust, on the fly, to atomic doctrine. 

Defensive Doctrine

Thanks to long-range, atomic-capable missiles and rockets, linear defenses were no longer applicable, and the same idea of small, dispersed units on the offense would have to be applied to the defense. Thus, mobility would be the key to defense in a limited nuclear war. The doctrine called for using a mobile, layered defense that accepted penetration but sought to canalize enemy incursions into a position to be destroyed — like an amoeba. Therefore, the Pentomic Division would be responsible for a 20,000-to-25,000-meter defensive area, deploying its five battlegroups with its tank and armored cavalry elements arrayed with two battle groups at the front, two behind them, and the fifth in reserve. This technique was used so that a unit could withstand an atomic strike but still provide ample conventional defense to repel a Soviet offensive in Germany.

After an atomic strike, Army planners assumed Soviet forces would exploit a breach in the defensive lines. The amoeba-like layered mobile defense would, then, seek to draw the Soviets into a desired kill zone, where they would be worn down and eventually destroyed by the American/NATO defenders. The key was maintaining sufficient combat power after the initial atomic strike — something a well-dispersed unit could ostensibly do.

If a defender could not be mobile, then digging in and preparing positions was paramount. Still maintaining wide dispersion, well-prepared earthworks could reduce the destructive power and danger of an atomic strike. Covered foxholes on reverse slopes offered the best protection for troops. Using alternate and supplementary positions for companies and platoons to shift to under fire was likewise vital as the defender drew the enemy advance into its kill zone, wearing it down before destroying it. 

Another area defense technique was a strong point — the “islands of resistance” defense — where commanders could move their various units as needed, like pieces on a chess board, to neutralize and destroy the opponent. This technique required three-to-five-mile gaps between battalions that were only covered by improved surveillance and fires. 

Mobile, dispersed units needed an immense amount of decentralized leadership. They also needed to be flexible enough in command and control to fight fluid battles on the nuclear battlefield. Companies and platoons were required to operate independently yet still be able to support each other on demand. This ideal required high-functioning and well-rehearsed communications plans on advanced communications gear — a critical failing of the American Pentomic concept. The Pentomic five-sided span of control was too expansive for contemporary command, control, and communications capabilities.

Offensive Doctrine

Offensive atomic doctrine was predicated on nuclear fire support, dispersion, speed, and mobility. Fire and maneuver would still reign supreme, and Army leaders believed that atomic weapons would make a strike at the adversary’s most vital point to blast through an enemy’s main line of defense. This approach would be the cheapest and most straightforward route to the enemy’s rear. However, that nuclear strike had to occur before the ground movement began because units could only shoot through the gap after the first strike.

This approach required a robust command and control system, not just to authorize the use of nuclear weapons, but also to choreograph the maneuver required to exploit their use. The Army needed to become a hyper mobile force that relied on greater mission-type delegation authority to lower echelons. Logistics also needed intense planning, with supplies dispersed and located deep in the rear of the main battle area. The idea was to pool logistical units at higher echelons and institute a rapid and mobile resupply system dispersed amid small, scattered supply points to reduce their signature and not invite a Soviet atomic attack. Seizing and retaining large bases and stockpiles within range of adversary weapons invited attack.

The End of the Pentomic Division

The catastrophic problem, however, was that wargaming and common sense revealed that any use of a tactical nuclear weapon on the battlefield quickly escalated and ended with the exchange of large numbers of nuclear weapons. Thus, any advantages a one-time use of nuclear weapons on the battlefield gained would be wiped out by the likelihood of nuclear escalation — a truism that Reagan and Gorbachev acknowledged some three decades later. 

Beyond this, the Pentomic Division was unable to meet the exacting requirements to take advantage of nuclear use. The change from triangular division structures with Regimental Combat Teams to the Pentomic Division and its Battle Groups produced units far less trained and capable. Training below the division level was mostly conventional, where units were encouraged to disperse more than normal but never enough for an impending nuclear attack. Furthermore, release authority for nuclear weapons remained at division command or higher. Besides the Pentomic airborne division, it remained too burdensome to have fully air-transportable divisions with an atomic capability that could also snuff out limited wars before they escalated into full-blown regional conflicts. The infantry and armored divisions required more sealift due to dual-capability requirements. Finally, a dual-capable force was considerably more expensive: The new division cost approximately 35 percent more per soldier than the triangular configuration. 

The Pentomic Division was supposed to achieve savings by substituting firepower for manpower and increasing the ratio of combat to support personnel. But it ultimately had less conventional firepower than before, augmented by nuclear fires to achieve better fire support. Likewise, the technology required to outfit the Pentomic Division was expensive and increased the division’s costs. The equipment on hand was insufficient to make the increased span of control effective: It simply did not provide the flexibility to fight in non-nuclear environments. Further, much of the equipment developed during the era had long been on the Army’s wish list, and the change to the Pentomic Division provided a method to fund that equipment. In 1962, the Army began returning to a flexible, modular triangular structure with three brigades that went into effect over the next few years.

Lessons for Today

Historical examples found in doctrinal developments of the Pentomic Era offer a lens through which to envision Ukrainian forces’ survival and success. The first, and most salient, is that leaders considering battlefield nuclear use must also consider that limited use is likely to escalate to large-scale nuclear war. In Ukraine, these dynamics do appear to have deterred serious consideration of tactical nuclear weapons use, even if local commanders may have talked about scenarios for nuclear employment. 

Beyond this, the intellectual project of developing organizational schemes and maneuver doctrine for the Pentomic Division offers some lessons on preparing to fight on an atomic battlefield. First, units must be comfortable with dispersion, which requires increased trust between commanders and subordinates. Second, task-organizing combat enablers at the lowest echelon will ensure they are where they need to be when the time comes for their use, especially after an atomic strike. Third, command and control must be rehearsed so that units can maintain dispersion in the defense and coordinate fires to cover dead space. Fourth, reconnaissance is paramount. In the 21st century, this includes drone assets to cover the wide gaps between friendly defensive units to bring devastating fires on any force that seeks to exploit those gaps. 

Most of this is common sense. By all accounts, modern forces have the equipment to disperse and mass when and where needed to exploit enemy weaknesses following an atomic strike. The Ukrainians are incredibly adept at mobile “shoot and scoot” tactics, relying on empowered non-commissioned officers thanks to NATO training. Since the beginning of the war, they have employed “hit and run” anti-armor tactics and now wage well-coordinated and dispersed offensives that include deep-guided rocket HIMARS strikes at Russian logistics. Ukrainian tactics also confirm U.S. Army Chief of Staff General James McConville’s modernization priorities. Still, should low-yield atomic weapons be used in Ukraine, their effects will not be contained to the battlefield.

The use of nuclear weapons, like any escalatory action in war, is wrought with uncertainty. Wars have a way of spiraling out of control. The use of a so-called “tactical” nuclear weapon in Ukraine would carry with it grave and unknowable consequences, furthering Russia’s status as international pariah at a minimum — and the outcomes are uncertain. The pentomic era also suggests that an ill-trained military will struggle to exploit or take advantage of a nuclear strike, raising further questions about the efficacy of any such action. The specter of nuclear conflict has returned to Europe. However, when looking back at lessons from the past, the logic underpinning nuclear use is suspect at best.

R.F.M. Williams is a doctoral candidate in military history at Ohio State University and a graduate fellow at the Mershon Center for International Security Studies. He previously served as a U.S. Army infantry NCO with three combat deployments in Iraq and Afghanistan. He can be reached on Twitter at @rfmwilliams.

Image: National Nuclear Security Administration 

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