It's the local maximum and a prisoner's dilemma in one. You'd need everyone to agree to change at the same time and they'll need to believe no neighbours will c

It's the local maximum and a prisoner's dilemma in one. You'd need everyone to agree to change at the same time and they'll need to believe no neighbours will conquer them by being able to afford more military. This gets even more rough when the available working/serving population gets smaller and any local gains are bigger relative to your neighbours.