US fires 4th round of retaliatory airstrikes against Iran-backed militants

This is the fourth round of American retaliatory airstrikes in response to a near-daily spate of attacks on U.S. forces in Iraq and Syria by Iran-backed groups. SUBSCRIBE to ABC News on YouTube: https://bit.ly/2vZb6yP Latest updates: http://abcnews.go.com/ Watch FULL EPISODES on Hulu: http://abcn.ws/3bzvQQn