Bush senior proclaimed a “new world order” after the quick and decisive victory in the 1991 Gulf War, thinking that a permanent international consensus to enforce norms of decency had been forged. Though that grand vision never came to pass, the notion that the United States and its allies were now free to project power to “do good” has remained intact.

This has coincided with a still-ongoing revolution in global communications technology. With the rise of network news channels that can broadcast far-away violence into American living rooms, and more recently of social media technologies that give voice to oppressed peoples in all corners of the globe, this environment has made it much easier for advocates of humanitarian intervention to make their case.

Realist arguments about national interests, unknown risks, and post-conflict reconstruction have proven far less able to sway Americans than are television images of humans being slaughtered. Whereas the victims of Idi Amin were statistics, those dying in the Arab Spring have faces, names, and Facebook accounts.