Direct proof of use
U.S. Marine Corps M198 155 mm howitzers were documented in combat use around Fallujah during the 2003 Iraq War. A U.S. Department of War photo caption identifies Marines from M Battery, 4th Battalion, 14th Infantry Regiment, 1st Marine Division RCT-1, firing an M198 to provide supporting and defensive fire for Camp Fallujah on October 21, 2004.
A separate DVIDS image caption identifies Marines from 4th Battalion, 14th Marine Regiment, firing a 155 mm M198 Medium Howitzer on an insurgent position from the outskirts of Fallujah during Operation al Fajr.
Sources: Department of War Camp Fallujah M198 Photo, DVIDS Operation al Fajr M198 Photo
Timeline
The strongest public record clusters in late 2004. On October 21, official imagery documented an M198 firing for Camp Fallujah support and base defense. During the November 2004 Fallujah assault, Marine Corps News reported that Battery M, 4th Battalion, 14th Marine Regiment fired 1,742 rounds from November 8 through November 19 in support of Marine and Army infantry units in the city.
That Marine Corps News account says the battery's mission changed from counterfire support for Camp Fallujah to battlefield fire support for Regimental Combat Team 1 infantry units as the operation developed.
Sources: Department of War Camp Fallujah M198 Photo, Marine Corps News Fallujah Cannoneers
Narrative
The documented operator was the United States Marine Corps, with Battery M, 4th Battalion, 14th Marine Regiment repeatedly appearing in the available public record. The howitzers served an indirect-fire role: defensive and supporting fires at Camp Fallujah before the main November assault, then fire support for infantry units fighting in Fallujah during Operation al Fajr.
The sources support use of the M198 as a U.S. coalition fire-support weapon in the Iraq theater, not Iraqi possession or insurgent use during the 2003-2011 war. Later Iraqi and Islamic State use of M198 howitzers belongs to the separate 2013 War in Iraq catalog context.
Sources: Department of War Camp Fallujah M198 Photo, DVIDS Operation al Fajr M198 Photo, Marine Corps News Fallujah Cannoneers