WASHINGTON — Twice in the last week, the United States has focused its vast manhunting machinery on tracking down and striking terrorist leaders in anarchic countries that for the White House once embodied the promise of the Arab revolutions across the Middle East.

A drone strike in Yemen killed Nasser al-Wuhayshi, the leader of Al Qaeda in the Arabian Peninsula, who had built a terror franchise feared in the capitals of the West. Days later, the Pentagon dispatched F-15 jets to Libya to kill Mokhtar Belmokhtar, who in 2013 planned the seizure of an Algerian gas plant in which 38 foreign hostages died. On Tuesday, it was still uncertain whether he had been among those killed in the attack.

The strikes may prove to be lasting victories for the mode of long-distance warfare embraced by President Obama, but the ultimate impact of killing terrorist leaders like Mr. Wuhayshi remains to be seen. The administration and its foreign allies have been unable to stem the chaos in Yemen and Libya, and hopes that a new democratic order could emerge after the fall of dictators have been reduced to far more humble goals.

In the 18 months remaining in Mr. Obama’s presidency, and with Qaeda and Islamic State fighters filling a power vacuum in both Yemen and Libya, the occasional killing of militant leaders might be the most the administration can hope to achieve in the two war-racked countries.