Even as President Obama lays out his vision of America’s obligations to civilians menaced by their own governments, the limits of what some are calling an “Obama Doctrine” are evident in his differing approaches to Libya and another Arab country in turmoil — Syria.

Ignoring Libyans in danger from Moammar Kadafi‘s forces “would have been a betrayal of who we are,” the president said this week in defending the U.S.-supported air war there. Yet the administration has made it clear that the military option is not “on the table,” in Syria, where dozens of protesters also have been killed.

A senior administration official confirmed that even milder measures, such as a tightening of U.S. sanctions, withdrawal of the ambassador to Syria, or other efforts at economic or diplomatic isolation are not being weighed.

Both countries are within reach of missiles and warplanes launched from U.S. ships in the Mediterranean. The difference in U.S. policy toward them is an example of Obama’s general approach to government — seeing policy in shades of gray rather than black and white.

But the administration has struggled to explain why it has intervened in one country and not the other. The explanation is difficult in part because it has less to do with humanitarian issues than with harder calculations of national interest…