The withdrawal of U.S. troops from Afghanistan signals the end of a war that involved close to 800,000 American service members. Defending against new threats will require the U.S. to replenish its all-volunteer force with fresh recruits — a task made harder by the dwindling number of Americans willing and able to serve.

The U.S. currently has 1.3 million active-duty service members. Due to attrition and retirement, the military needs to find more than 150,000 new recruits every year to meet its overall "end strength” goal. In 2020, the Army, Navy, Air Force and Marines all hit their annual recruitment goals — but those figures were distorted by a historically weak job market, as active-duty service members delayed plans to re-enter the civilian sector, reducing the need for new hires. Last year’s target of 61,200 new Army enlistees, for instance, was 20% lower than in 2018, when the Army failed to meet its goal.

Recruiting isn’t easy. At least 70% of Americans between 17 and 24 are ineligible for military service due to obesity, mental-health issues, past drug use, criminal records or lack of a high school degree. Overall, only 13% of young adults express a positive propensity to serve, with women about half as likely as men to consider enlisting.