After witnessing the carnage in Ukraine, the U.S. and its partners are racing to rearm — beefing up defense budgets, securing supply chains and expanding arms production. To deter the likes of Russia or China, though, they will have to devote the same energy to another task: finding troops to man those weapons.

For years now, many advanced militaries have struggled to meet recruiting targets. The U.K. and Japan have failed to hit quotas for a decade; the latter barely reached half its target last year. Germany’s military needs to add more than 20,000 soldiers by 2031. The U.S. Army has stumbled since 2022, missing its goals by 25% and 17% over the past two years; this year the target has been lowered. The U.S. Navy — which would play a central role in any conflict in the Pacific — expects to fall short by more than 4,000 recruits.

The factors driving the recruiting crisis vary by country. Still, some broad pressures are clearly at work. In Asia and Europe, demographics are inexorably shrinking the pool of candidates. South Korea, the most extreme example, largely relies on conscription to provide the 200,000 recruits per year needed to fend off the North’s bigger army. In two decades, there may be only 125,000 young men to fill those spots.

Even in countries with growing populations, as in the U.S., too few youth meet the physical and mental qualifications for service — and, especially when civilian jobs are plentiful, actually want to join. The U.S. is also contending with a drop in the number of young men looking for work, as well as a decline of trust in the military, for reasons that aren’t well understood.

So what is to be done?

Changes to the process can help. Using more talented officers as recruiters and providing bigger bonuses have been shown to work. Having top brass focused on the issue is crucial, as is better marketing. Offering more choice in where to serve could convince some candidates. Allies should analyze and share data on which strategies are working and why.

It is important not to compromise on quality: While preparatory boot camps have gotten thousands of poor performers in shape to join the U.S. Army, commanders must watch carefully how those soldiers do over time. It’s also worth examining whether enlistment standards still make sense for all recruits or whether, say, more medical waivers can be issued for low-risk conditions. Some changes may be dictated by need: Japan, for instance, is considering relaxing fitness requirements for cybersecurity officers.

Aging societies may require more drastic measures. The “selective compulsory” system of conscription used by Norway and Sweden, where all citizens are required to register but only the needed percentage are chosen to serve, may work for some countries.

A more obvious strategy is to enlist more women, which will require addressing widespread problems with discrimination, sexual harassment and lack of child care. Programs such as those in the U.S. and Australia that offer an accelerated pathway to citizenship for immigrants who serve should be adopted more widely.

Although automated weaponry is progressing rapidly, killer robots are unlikely to replace soldiers anytime soon. In the short to medium term, boots on the ground will still be needed, not least to pilot uncrewed vehicles and man defenses against them.

On the other hand, top brass should continually look at whether shifting more tasks to civilians or contractors could reduce the number of uniformed personnel needed without adverse effect. Simply adopting more efficient civilian software for payroll, logistics and other tasks could lead to additional labor savings.

Finally, militaries should put as much effort into retention as recruitment. Bonuses, better housing and child care, and faster promotion pathways could help preserve a highly trained, highly motivated cadre of experienced troops. Given the effort required to get those citizens into uniform, they should be given every reason to keep wearing it.