U.S. military and coalition partners have begun airstrikes on Syria. ABC News:

American airstrikes against ISIS targets are under way in Syria, according to a Pentagon official.

"I can confirm that U.S. military and partner nation forces are undertaking military action against ISIL [ISIS] terrorists in Syria using a mix of fighter, bomber and Tomahawk Land Attack Missiles," Pentagon Press Secretary Rear Admiral John Kirby said. "Given that these operations are ongoing, we are not in a position to provide additional details at this time. The decision to conduct theses strikes was made earlier today by the U.S. Central Command commander under authorization granted him by the commander in chief. We will provide more details later as operationally appropriate."

Twitter actually had an earlier heads-up on this:

No small irony: The warship supporting the U.S. airstrikes is the USS George H. W. Bush

Update: These strikes are not unilateral. According to BreakingNews.com, Jordan, Saudi Arabia, Bahrain, Qatar and UAE are participating.