On October 8, the USS Abraham Lincoln and its accompanying strike group returned to the United States after 320 days at sea. The deployment—a near record for the modern American military—had become controversial. As the group’s ships went month after month without a port call, conditions deteriorated precipitously. Sailors reported supply shortages, plumbing issues, and water contamination. Morale dropped markedly; eight sailors even tried to commit suicide. By the time the ships finally left the Middle East, their struggles had made headlines across the country.

The travails of the Lincoln are hardly the only story about the U.S. military to generate alarm since the war in Iran began last February. Military analysts have expressed concern about other carrier groups’ lengthy deployments. They have fretted about the Trump administration’s decision to pull soldiers and weapons stationed in the Indo-Pacific out of the region and position them in the Middle East. Perhaps most of all, they have worried as the United States drains its stockpile of sophisticated munitions.

These problems all share the same root cause: the U.S. military is not built for the near-constant use it has experienced for decades. Ever since the Cold War, American defense planners have structured the U.S. armed forces to be ready to fight two major conflicts simultaneously. In practice, that means Washington has constructed a force that is designed to win short, sharp wars. For that reason, the military does not have enough units, weapons, or personnel to handle never-ending conflicts and simultaneously use its forces around the world to coerce enemies and reassure partners.

American officials are now trying to modestly increase the size of the military. More important, they are hoping that huge infusions of money will increase the capability of the existing armed forces. But making these changes will be exceptionally challenging—and they are unnecessary. The United States is a highly secure country with many capable and wealthy partners. It does not need a defense apparatus capable of fighting everywhere all at once. Instead, it would be much better off focusing its attention on a smaller set of issues and scaling down accordingly.

The U.S. nonnuclear forces, which consume more than 80 percent of the defense budget, are structured and sized for particular contingencies—currently, a clash with China, Russia, or both nearly simultaneously. The military’s weapons are therefore designed for maximum lethality and survivability in what military planners expect to be an unbelievably dangerous environment. To that end, the Pentagon has emphasized purchasing precise, long-range, and stealthy weapons. These are supposed to kick open the doors of enemy defenses and allow other expensive advanced platforms, such as F-35 fighter jets or B-2 or B-21 bombers, to deliver large quantities of slightly less sophisticated but still quite lethal precision weapons.

Washington has never expected these wars to last long. Instead, it imagines that under the fierce pressure such conflicts would generate, the adversary will cry uncle or the war will escalate to nuclear threats or—worse yet—nuclear use. During the Cold War, for example, the United States and its allies planned for an extremely intense, roughly 30-day conflict against the Soviet Union. Today’s military planners have probably made similar assumptions about a war with Beijing or Moscow. Highly capable........

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