Analysis: Jim Muir, BBC News, northern Iraq
The American air attacks, the first of their kind in Anbar province, signal that Washington has crossed a line that it itself drew.
It has long had a standing request from the outgoing Iraqi government to use its air power against IS in all areas. But until recently, it made it clear it would only do that once a new, inclusive government is formed in Baghdad, with full Sunni representation.
That hasn't yet happened, though intensive efforts are under way to produce a new cabinet in the coming days.