Three years after pulling out the last US troops from Iraq, president Obama ordered warplanes to bomb advancing Islamist fighters in the north of the country.
The militants are threatening both a major Kurdish city and tens of thousands of members of a religious minority trapped in the Sinjar mountains, fearing for their lives.
Several US strikes have already taken place, and while Britain has ruled out military action, it will make humanitarian aid drops.
ITV News Washington correspondent Robert Moore reports on the American action:
Kurdish Peshmerga fighters have emerged victorious in the battle for Mosul Dam, but the question is - what will their next move be?
An ITV News team at Mosul Dam was with victorious Kurdish Peshmerga fighters at the complex earlier today.
ITV News joined a Kurdish unit fighting Islamist militants near Irbil and found them using tanks from the 1950s and short on ammunition.