The commander of a U.S. military post near the center of a coronavirus outbreak in South Korea delivered the sobering news to his troops in warriors’ terms: “We had a breach in our perimeter.”

A soldier at the post, Army Garrison Daegu, had tested positive for the virus Tuesday — the military’s first active-duty case — and the commander, Col. Edward Ballanco, told the troops in a video address, “Let’s regroup and attack the virus.”

The battle plan he announced was aggressive: Close the office buildings the infected soldier used. Have “clean teams” disinfect anywhere the soldier had been. Quarantine whoever he had come in contact with. At the post gates, screen everyone for fever, troops and civilians alike. Tell many civilian workers to stay home. Close the post’s schools, golf course and bowling alleys. Cancel upcoming social events like the father-daughter dance.

The coronavirus threat may still seem distant to much of civilian America, but it has been a clear and present danger for the military almost from the start. The U.S. has more than 75,000 troops stationed in countries that are experiencing outbreaks, including South Korea, Japan, Italy and Bahrain.

Several U.S. bases sit next to cities where the virus is spreading, and they are intertwined with local communities, employing numerous civilian workers and housing many troops off base. A civilian worker at another Army post in South Korea tested positive Thursday.

And in any kind of crisis, the nation’s first resort is often to call on the military’s capabilities and resources, as it did when infected Americans evacuated from a cruise ship in Japan were quarantined on two air bases.

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As the virus spreads around the globe, Defense Secretary Mark Esper told Congress this past week, the “first priority is protection of our people, both service members and families, and then make sure we protect our ability to accomplish our mission.” Whatever tasks troops may be called upon to perform, from erecting field hospitals in overwhelmed communities to transporting patients in cargo planes filled with isolation pods, military leaders have emphasized that they can be accomplished only if the troops are healthy.

This past week the military took several defensive steps against infection, curtailing some operations and restricting leave.

But like the global business supply chain, the U.S. military relies on constant movement of people and material among far-flung countries, so force protection can come at a cost. Steps meant to stave off or contain the virus may also compromise training and readiness.

That is already happening in South Korea, which has reported more than 1,700 cases of the viral illness, known as COVID-19. United States Forces Korea announced Thursday that it was canceling annual joint military exercises with the South Korean military.

But officials said this past week that another joint training mission, in Thailand, had been allowed to go ahead.

“There is a balance we have to strike, and it’s not easy,” said Dr. Jonathan Woodson, who was assistant secretary of defense for health affairs from 2010 to 2016, and who coordinated the military’s response to the Ebola outbreak in 2015.

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For as long as there have been armies, they have been menaced by the ready spread of diseases like typhoid, cholera and influenza that could tear through the ranks of troops in close quarters.

Even in wartime, illness and noncombat injuries “have claimed far more soldiers than gunshot wounds,” Woodson said. “That forced the military to create a robust public health system. You have to take care of the troops, or you have no one left to do the fighting.” In the Army’s dawning days, George Washington ordered all his soldiers to be inoculated against smallpox.

But health concerns have always competed with other priorities, Woodson said, and there is often debate over whether they should curtail operations, as they have in suspending the major exercise in South Korea.

“A lot of time, money and people are put into these exercises, and there are always people who want them to go forward,” he said. “But I think the leadership is being properly informed about the larger consequences.”

In many ways the military is better positioned than the civilian population to respond to a global outbreak. The armed forces are a young, healthy population that has universal health care. Bases with gates can easily limit access. Commanders have far more authority than civilian leaders to impose quarantines and vaccinations, close facilities and order troops to stay away from public gatherings.

“It’s the ideal public health environment, but it’s a double-edged sword,” said Carol R. Byerly, a former research historian for the Office of the Army Surgeon General. “You have great resources, great monitoring, but the needs of the military can also make the military more vulnerable.”

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She noted that during World War I, the deadly 1918 flu pandemic was made worse by the mobilization of tens of thousands of young men who were concentrated in cramped Army training barracks.

The Army’s expert on communicable disease at the time, Dr. Victor Vaughan, visited Camp Devens in Massachusetts and was shocked by what he saw: “Hundreds of stalwart young men in the uniform of their country coming into wards of the hospital in groups of 10 or more. They are placed on the cots until every bed is full, yet others crowd in. Their faces soon wear a bluish cast; a distressing cough brings up the bloodstained sputum. In the morning, the dead bodies are stacked about the morgue like cord wood.”

Vaughan and other military health officials appealed to President Woodrow Wilson to suspend the draft and stop sending crowded troopships to France. But generals seeking more troops for the trenches won the argument. The War Department later estimated that 35,000 U.S. soldiers died of influenza before reaching Europe.

“You’ve always had competing priorities,” Byerly said. “Medical officers are often seen as the mother hens of the military, and they are often seen as a burden and not listened to.”

She said the military seemed to be reacting prudently to the coronavirus outbreak, but she warned that much was still unknown.

“It’s always important to remember the history of infectious disease isn’t over,” she said. “We’ve lived in a golden age without a real pandemic, but we can’t get cocky.”

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In February, the military began executing a plan it developed in 2005 to battle a pandemic. The plan provides a blueprint for helping with evacuation and quarantine of patients, assisting with supplies and medical care, and, if needed, providing manpower to keep the nation’s critical infrastructure running.

As the threat levels laid out in military plans have been raised in recent weeks, more restrictions have been imposed. In several countries, including Bahrain, military schools have closed and students switched to online classes. At an Army post in northern Italy, theaters, gyms, day care centers and chapels have also been closed. Troops in South Korea were told Thursday to cut all nonessential off-base activity.

Some Navy ships near infected ports have been ordered to stay at sea at least 14 days as a safeguard. Troops stationed in the Middle East and Central Asia were told Thursday they could no longer take leave or liberty.

“The situation is not going to cure itself overnight,” Maj. Gen. Roger Cloutier said in a video briefing to troops at the base in Italy. “In fact, it could get worse before it gets better.”

Ballanco, the post commander at Daegu, said in his video message that the infected soldier, a 23-year-old man, appeared to be doing well. Officials at the post did not respond to requests for further information.

In his address, Ballanco warned that while the Army would remain vigilant, “we have to watch out about going overboard.”

“The Army has to continue to function,” he said, adding, “If we have doubts, we put you in quarantine, but eventually, if 30% of our base is quarantined, we’re going to face some other problems.”