As Russia confronts Ukraine, the definition of modern military intervention hangs in the balance.

We focus too much on military force instead of considering economic, technological, social, and political structures.

Kyiv needs to show Russians that the war is not worth fighting.

Whether we wish it or not, we are again in the world of war.

U.S. leaders need to talk to the Russians, not threaten them.

The country’s young people have been forced to make decisions much tougher than most adults are capable of. I feel some small sense of common cause.

Conflicts, though typically started easily, can be brutal, intractable, and difficult to end.

He is threatening to invade Ukraine because he wants democracy to fail—and not just in that country.

Anything helps—we shouldn’t overthink it. But we should still, well, think it.

Did Obama’s refusal to strike Syria really give Putin the green light in Ukraine?