The U.N. Security Council on Wednesday unanimously approved a historic resolution aimed at ending the flow of foreign extremists to the world's conflicts. President Obama thanked the council but warned that "a resolution alone will not be enough." Julie Jacobson/AP hide caption

itoggle caption Julie Jacobson/AP

A photo released by the U.S. Navy shows the guided-missile cruiser USS Philippine Sea launching a Tomahawk cruise missile against Islamic State targets in Syria on Tuesday. Eric Garst/U.S. Navy/EPA/Landov hide caption

itoggle caption Eric Garst/U.S. Navy/EPA/Landov

Members of Jabhat al-Nusra clean their weapons, in Aleppo in December. The Islamist rebel group has become an increasingly powerful force in Syria's civil war. Ahmed Jadallah/Reuters /Landov hide caption

itoggle caption Ahmed Jadallah/Reuters /Landov