As the U.S. role rapidly evolved, U.S. and Syrian Kurdish commanders began to coordinate air and ground operations far more closely than previously disclosed. A Syrian Kurdish general in a joint operations center in northern Iraq delivered daily battlefield intelligence reports to U.S. military planners, and helped spot targets for airstrikes on Islamic State positions.

In contrast to the lengthy legal debate over U.S. aid to rebels fighting the Syrian regime, U.S. airdrops of weapons to Kobani got a swift nod from administration lawyers—a sign of its importance to the administration.

The change in thinking over the fate of one city, described by U.S., Kurdish, Turkish and Syrian opposition officials, shows how dramatically U.S. war aims are shifting. After Islamic State made Kobani a test of its ability to defy U.S. air power, Washington intervened more forcefully than it had initially intended to try to stem the group’s momentum.

In doing so, the U.S. crossed a Rubicon that could herald a more hands-on role in other towns and cities under siege by Islamic State at a time when some U.S. lawmakers question the direction of American strategy and warn of mission creep.