There is no lack of U.S. national defense challenges.

China continues to modernize and expand its military, routinely using its burgeoning might to intimidate its neighbors—most recently with massive aircraft incursions into Taiwan’s air defense zone.

Meanwhile, President Biden has proposed a defense budget that doesn’t even keep pace with inflation and cuts the military by 5,000 people.

Analysts worry the U.S. is falling behind in such key military technologies as hypersonic missiles, quantum computing and artificial intelligence. Additionally, the disastrous departure from Afghanistan has raised concerns that the country may once more become a hothouse for global terrorism.

>>> Improving America’s Long-Term Military Recruiting Outlook

But the biggest challenge might be the one that nobody is talking about: The Pentagon’s difficulty in attracting enough qualified volunteers to serve in the armed forces.

If you thought it would be easy for a nation of 330 million people to attract 160,000 volunteers annually, you would be wrong.

The Army missed its recruiting goal in 2018 and has since struggled, and other services are experiencing difficulties too. And odds are that recruiting will get even more difficult in the coming years, to the point where the military may consistently fail to meet its goals.

To get a sense of future recruiting difficulty, analysts look at trends pertaining to demographics, the economy, disqualifying factors, veteran influence, future value of incentives and the public’s view of military service.

Spoiler alert: Every one of these indicators is either trending negatively or remains stagnant.

The key age bracket for recruits—18-24 years old—is projected to remain constant, hovering around 31 million through 2040, while the overall U.S. population grows, mostly in older age segments. As America ages, the opportunities for young people will make military service less attractive.

Increased unemployment normally leads to greater recruiting success, but economists forecast a gradual return to historic low unemployment, offering no relief for recruiting efforts.

Exposure to veterans has been linked to a greater propensity for individuals to volunteer, but the number of veterans in the U.S. population is projected to decrease by 1.7% per year, declining 17% from now to 2030.

As for disqualifying factors, youth obesity is projected to hit 24.2% by 2030, and the mental illness rate among youth hit 26.3% in 2018 and is expected to climb further. This further diminishes the pool of individuals qualified to serve in the military.

But there’s more.

The public’s confidence in the military—usually very high—has been slipping, dropping nine percentage points in the last decade. The chaotic withdrawal from Afghanistan will probably accelerate that trend.

Every year, more high school students go directly to college. Assuming that trend continues, fewer individuals will be available to join the military after high school.

Finally, one of the main incentives used to motivate individuals to join the military, the GI Bill, which provides tuition for college, will be diminished in value if progressives in Congress get their wishes to make community college free and to forgive student debt.

>>> What to Expect When You're Expecting a National Defense Strategy

Thus far, the Defense Department has chosen to view this issue through a narrow year-to-year lens, furiously manipulating bonuses, incentive programs, and the number of recruiters to achieve annual goals while failing to recognize the problem is getting harder each year.

Further, because each military service is expected to solve, on its own, what is legitimately an all-service, national problem, progress is slow and fragmented.

Fortunately, there are solutions if America approaches this issue seriously. By acting to reverse trends that are disqualifying youth from serving, reimagining existing recruiting tools, and engaging America’s youth earlier and in a more comprehensive manner, we can avoid this problem and preserve America’s national security.

Congress and the executive branch can choose to do nothing and wait till this is a full-blown crisis, or they can begin now to prepare America for what promises to be an increasingly dangerous and challenging future.