The limits of President Obama's power

President Obama's foreign policy has been characterized by ambiguities: a shape-shifting enemy, al-Qaida, continues to threaten violence around the world. U.S. forces increasingly use drones and missiles to kill at great distances, sometimes with vague rationales and innocent deaths occurring as a result.

Chaos in Iraq and feckless allies in Afghanistan threaten to undo the gains made by years of American sacrifice. The military prison at Guantanamo Bay has survived Obama's promises to shut it down. Allies have been dismayed to discover that the U.S. government was spying on them. The president's "red line" in Syria turned out to be more abstract than the term suggested.

And now, the crisis in Ukraine presents the Obama foreign-policy team with an unambiguous challenge: an expansionist Russia, asserting a right to inject itself into the affairs (and across the borders) of a smaller, sovereign neighbor.

The populist uprising that forced out Ukraine's president — or the unconstitutional coup, in Russia's view — crossed what turned out to be a red line for Putin. His reaction was swifter and surer than Obama's, and may also prove to be more dangerous. How Obama responds to Russia in the coming weeks may be a crucial test of his presidency.

What's the state of America's foreign policy under President Obama? Will he leave to his successor a legacy of indefinite, open-ended hostility against an ill-defined enemy? Will every president after this assert a right to launch attacks in other countries as frequently as Obama has done?