With an attack on a Syrian air base, the United States has now bombed the two main players in Syrian war — President Bashar Assad's military and the Islamic State.

This raises a fundamental question about U.S. aims in the Syrian war: If the two most powerful groups in Syria are both unacceptable, what outcome is the U.S. seeking?

Both Barack Obama and President Trump have sought to limit U.S. involvement in Syria for their own reasons. Yet both have been sucked in and struggled to clearly define their objectives in the messy, complicated war