This is an issue regarding their management approach.
Take the French Foreign Legion, for instance—it is worth looking into. In fact, many countries, including the United States, use incentives such as citizenship or permanent residency to attract foreign nationals to military service. The difference is that they typically integrate these individuals into mixed units, whereas the French Foreign Legion operates as a distinct, independent force.
For countries facing a shortage of citizens serving in the military, this strategy is highly effective. However, it requires appropriate management methods and measures; money alone cannot solve the problem.