The NYT reports that "as the tension mounted in Washington" the catalyst for Obama's decision was sudden developments surrounding the Kurdistan capital, Erbil. "Kurdish forces who had been fighting the militants in three nearby Christian villages abruptly fell back toward the gates of the city, fanning fears that the city might soon fall. By Thursday morning, people were thronging the airport, desperate for flights out of town. "The situation near Erbil was becoming more dire than anyone expected," said a senior administration official, speaking on the condition of anonymity to describe the White House's internal deliberations. "We didn't want another Benghazi."
The reason for this is that the US has an embassy in Erbil: that falling to ISIS would be the supreme punch in the gut for an administration whose foreign policy has become the butt of all global jokes.