By 2004, the province was in full-scale revolt. Al-Qaeda in Iraq (AQI) became the province's main Sunni insurgent group, and turned the provincial capital of Ramadi into its stronghold. Several battles, like Battle of Ramadi (2004), First Battle of Fallujah (April 2004), Second Battle of Fallujah (November–December 2004), Second Battle of Ramadi (2006), ravaged the region as the Iraqi insurgents struggled against the American occupiers for control of Anbar. During the first four years of Operation Iraqi Freedom, the Anbar Province was the deadliest province for American service members, claiming approximately one-third of American fatalities. Part of its significance came from the fact that the western Euphrates River Valley served as an important infiltration route for foreign fighters headed to Iraq's heartland from Syria.