WASHINGTON — One of the hallmarks of the United States military is its ability to project power around the world, often under the banner of slogans intended to strike fear in its adversaries. “Ready to fight tonight” for U.S. troops in South Korea; “America’s 911” for the Marine Corps expeditionary units at sea; the list goes on.

But now the foe is a novel coronavirus, and it has struck deep. More than 1,200 military personnel and their family members are affected, disabling a talisman of American military might — a nuclear-powered aircraft carrier — and leaving the Defense Department virtually at war with itself over two competing instincts: protecting troops from the virus and continuing its decades-old mission of patrolling the globe and engaging in combat, if ordered to do so.

The Navy is thus far refusing to completely evacuate an aircraft carrier where 93 service members have been confirmed to be infected with the coronavirus. Defense Secretary Mark T. Esper has put himself on the side of business as usual in maintaining readiness while also saying that force protection is a top priority. President Trump, for his part, threatened a familiar foe, tweeting on Wednesday that Iran would “pay a very heavy price” if its proxies attacked American troops or assets in Iraq. Other Defense Department officials continued to insist that the aircraft carrier, the Theodore Roosevelt, remain ready to carry out its missions.

The commander of the Roosevelt, Capt. Brett E. Crozier, pointed out in a strongly worded letter that “we are not at war.” That statement raised questions from the Pacific to the Pentagon of what was so important about the aircraft carrier’s presence off the coast of Guam that the Defense Department could not evacuate the ship and do a deep cleaning, as suggested by Captain Crozier.