So if you ask, Why do American troops not do what Athenian troops did--march into foreign cities and kill all male citizens--part of my answer is this: You just can't do business with people while executing all their male citizens, and increasingly we do business with people everywhere.

But it isn't just economics that makes the modern world more broadly interdependent than the ancient world. War between Persia and Greece was basically zero-sum--there would be a winner and a loser. (This is what made relations among Greek cities more non-zero-sum.) But since then technology has made war itself more and more non-zero-sum. In a nuclear war, both players lose. The Greeks could afford to hate the Persians, but Americans can't afford to hate the Russians--or, at least, they can't afford to act on such hatred.

Of course, strategically refraining from bombing someone is not the moral equivalent of empathizing with them. Even in an interdependent world, most people feel the suffering of a nearby friend more deeply than the suffering of a transatlantic stranger. But this gap is shrinking, too. First, because more people have friends across the ocean, what with business travel and online chat groups and international organizations. Second, because TV lets us see the Russians up close and personal, in a way the Greeks couldn't see the Persians. Though that by itself is no guarantee of empathy, it does make it harder to caricature foreigners as monsters, and easier to see them as basically like us, with the generic human set of hopes and fears.

Some people may find it disheartening to think that the "expanding circle" of moral concern has been driven mainly by crass self-interest as mediated by advancing technology. Personally, I find this cynical theory oddly comforting. Given how stubborn human self-interest is, and how inexorable technological advance has been, the theory suggests that the dawning of this most basic moral truth--the fundamental equality of everyone--has been in the cards all along. And presumably, as technology continues to advance, weaving the world into an ever-more-interdependent whole, we'll have to take this truth more and more seriously.