The strong do what they can and the weak suffer what they must. So said Thucydides in his account of the Peloponnesian War. And though these words were written nearly 2,500 years ago, the idea that a state’s power determines its conduct remains one of the canons of modern international relations theory.

To be sure, strong countries often do restrain their impulse to conquer or coerce weaker states. They may determine that the benefits are low, even if the costs and risks are also. And they may worry that undertaking a foreign adventure in a faraway place will distract them from more proximate security threats. In short, they prioritize. They choose.