A good many people across the political spectrum—including some members of the Obama administration—are pressuring the president to intervene militarily in Libya. Much of the commentary has focused on establishing a no-fly zone, but there have been calls as well for enforcing a no-drive zone, or for arming or otherwise assisting regime opponents.

Those making this case appeal to a mixture of morality and realpolitik. They argue that by intervening we will prevent the slaughter of innocents and at the same time demonstrate...