 At the beginning of May, coalition forces estimated that there were 200-500 militants still present in Karbala, 300-400 in Diwaniyah, an unknown number still left in Amarah and Basra, and 1,000-2,000 still holed up in the Najaf-Kufa region.Coincidentally, the offensive against Fallujah started on the same day that the Shia uprising began.In response to the killing of the four Americans on March 31 and intense political pressure, the U.S. Marines commenced Operation Vigilant Resolve.