A 1918 warning sign at the Naval Aircraft Factory in Philadelphia that begins, “Spanish Influenza has endangered the prosecution of the war in Europe.”

Pandemic Effects on War and Society

Military History

In 541, Byzantine Emperor Justinian I was well on his way to reconquering the western half of the Roman Empire, but a microbe stopped him.

Yersinia pestis, better known as the bubonic plague, made its first appearance in recorded history, traveling along with shipments of Egyptian grain across the Mediterranean world. During its run, it killed up to 100 million people, weakening Roman armies so much that they lost control of Italy and never regained it.

The Justinianic Plague is just one example of pandemic effects on military history. Over the millennia, wars have contributed to turning localized outbreaks into pandemics, which have altered both the courses of wars and the ways in which they’re fought.

“You can't separate war from disease,” says Sarah Douglas, a lecturer on military history at Norwich University. “Sick soldiers and sailors are less effective in combat than healthy ones if not rendered unable to fight entirely through illness or death. If disease was present, it was always a factor—if not a deciding one—in a military’s victory or defeat.”

As the world wrestles with the effects of the COVID-19 pandemic, a look at the past suggests how both modern societies and militaries might change in the years to come.

Armies as Superspreaders of Historic Pandemics

Two factors make armies efficient vectors for spreading disease: contact and travel. “You're having groups of people in close proximity for prolonged periods of time,” Douglas says, “and they're moving across distances.”

A prime example is Napoleon’s 1812 invasion of Russia. While people often blame the rigors of the Russian winter for the expedition’s failure, another culprit was typhus, a disease transmitted through body lice. It wiped out 80,000 soldiers in just the first month of Napoleon's campaign.

In contrast, disease prevention programs helped other armies to win wars. At Valley Forge, while Baron von Steuben drilled George Washington’s soldiers in Prussian battle tactics, he also taught them basic principles of sanitation. They restructured their camps, moving latrines downstream from dining areas to avoid contaminating drinking water. The Continental Army also reduced the incidence of smallpox through a program of crude inoculations.

The Black Death

Perhaps the deadliest historic pandemic was the bubonic plague outbreak of 1348-51. The so-called Black Death vividly illustrates the complicated, symbiotic relationship between wars and pandemics.

War brought the plague to Europe. An outbreak that began in central Asia spread westward, through trade along the Silk Route, to the Turkic khanate of the Golden Horde. In 1346, when the Horde besieged the Genoese merchant city of Caffa on the Black Sea, it flung bodies of plague victims over the walls of the city. Fleeing merchants brought the contagion to Italy’s ports.

As the Black Death rampaged across Europe, it killed up to 50% of the population. People fled cities, governments stopped operating, and trade slowed to a trickle. “A lot of port cities shut their ports and refused incoming ships because they were afraid of the transmission of the disease,” Douglas says.

The plague interrupted government and commerce as well as war. For two years, England and France paused their campaigns in the Hundred Years’ War.

When the conflict resumed, it was in very different fashion. In a forthcoming book, Douglas examines two English invasions of France waged by Edward III before and after the pandemic: the 1346-47 Crecy-Calais campaign and the 1359-60 Reims campaign:

  • In the earlier expedition, the king transported 30,000 soldiers to France at once, in a fleet of 470 ships.
  • By 1359, not enough sailors were available, which meant smaller ships and more trips. The fleet had to cross the English Channel five times to pick up and drop off soldiers.

The Plague’s Pandemic Effects on Armies

The immediate results of a pandemic are devastating, but over the long term, some pandemic effects can be positive. Douglas says. “What you really see is that humans are resilient and find a way through.”

Far-reaching social and economic changes emerged in the aftermath of the Black Death. Cities introduced public health measures such as temporary quarantines. England reformed its customs system to triple the Crown’s revenue. Italian city-states such as Florence, Genoa, and Venice grew more independent of the Holy Roman Empire, reformed their banking systems and funded the Renaissance.

Militaries transformed to adapt to a world of diminished manpower as well. “You had people who wanted to go to war, but they didn’t have as many soldiers to do it,” Douglas says. “Labor-saving devices in the realm of military affairs seemed to be the result.”

Ship Rigging  

In the early 1300s, the dominant ship design was the cog, which featured a single mast and sail that required operation by 30 to 40 sailors. By the end of the century, the expansion of rigging allowed larger ships to run with a third fewer sailors.

As a consequence, Douglas says, “Europeans started to expand more into the oceans, which contributed eventually to the oceanic travel they undertook in the early modern era.”

Gunpowder Weapons  

Cannons existed before the Black Death, but in the words of one chronicler, all they did was scare horses. In the pandemic’s aftermath, with fewer soldiers on the field, armies compensated by making artillery more powerful. As Douglas notes, “Why raise an army of 1,000 men to storm a castle if you can raise 500 and use cannons to knock the walls down?”

Professional Soldiering  

Before the Black Death, English kings had raised armies through conscription, directing feudal nobles to fill quotas of peasants. Afterward, farms could spare few peasants.

As a result, in his 1359 campaign, Edward III filled his ranks with paid recruits. Indentured contracts specified how long soldiers would work, how much they would earn, and even how they would be compensated if their horses were lost in battle.

Together, Douglas says, the changes laid the groundwork for what historians call the Military Revolution, a transformation of European armies that, by 1650, enabled them to establish global empires.

COVID-19 Pandemic Effects on Today’s Militaries

An important lesson from the Black Death is that societies did not seek to return to pre-pandemic status quos, Douglas argues. “The societies that thrived—those who proved most successful politically, socially, economically, and/or militarily in subsequent decades—were those that embraced the changes brought about by the pandemic.”

Likewise, she believes, today’s novel coronavirus pandemic could lead to profound changes long after vaccines bring the virus under control. Many have begun.

  • Health care systems are speeding their embrace of telemedicine, international research cooperatives, and new biotechnologies such as messenger RNA-based vaccines.
  • Businesses are shifting their best practices to include work-at-home positions, while retail is moving more quickly to online ordering, curbside pickup, and home delivery.
  • Governments are restructuring their approaches to social welfare, social policing, and social safety nets.

In the military sphere, Douglas foresees less emphasis on boots on the ground or planes in the air and more on weapons that can strike from a distance. The U.S. Navy, for example, is developing solid-state lasers, electromagnetic railguns, and hypervelocity projectiles. “You're trying to keep yourself farther away from the enemy while also instilling maximum damage,” she says.

The pandemic might also accelerate the movement of war from the physical realm to the digital, Douglas says, with different kinds of casualties. “Rather than invading a country, isn't it safer to hack it and shut down its infrastructure?” she asks. “Maybe future acts of war will be non-physical and directed assaults like cyber attacks, which are not going to involve massive bombing campaigns. The impact will be more economic and political, as opposed to having a direct human cost.”

Military History: More Than Wars and Guns

Pandemic effects on warfare illustrate that military history isn’t just about the past; it offers insights into today’s challenges. It also covers more than just battles and weapons. It has shaped and been shaped by geographic, political, economic, and social forces.

The online Master of Arts in Military History program at Norwich University helps aspiring military historians explore those connections. Courses range from specific places and periods such as China or 19th century America, to societal issues, such as race, gender, and Western legal traditions.

Norwich University, founded in 1819 as America’s first private military college, holds a proud place in America’s military history. Today, it prepares historians of the future.


Recommended Readings

Pandemics Throughout History
What Is Historiography? The Importance of Writing About Military History
Why Study Military History? Understanding Career Options


The History of Plague — Part 1, Journal of Military and Veterans’ Health
Napoleon Wasn’t Defeated by the Russians, Slate.com
Friedrich von Steuben Arrives at Valley Forge, History.com
Biological Warfare at the 1346 Siege of Caffa, Centers for Disease Control and Prevention
Black Death, History.com
Myths of Military Revolution: European Expansion and Eurocentrism, European Journal of International Relations
Report on Navy Laser, Railgun and Gun-Launched Guided Projectiles, U.S. Naval Institute News

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