As a major general, Mattis commanded the 1st Marine Division during the 2003 invasion of Iraq and the Iraq War.
Mattis played key roles in combat operations in Fallujah, including negotiation with the insurgent command inside the city during Operation Vigilant Resolve in April 2004, as well as participation in planning of the subsequent Operation Phantom Fury in November.

In May 2004 Mattis ordered the 3 a.m. bombing of what he alleged was a suspected enemy safe house near the Syrian border, but was in fact a wedding party and resulted in the deaths of 42 civilians, including 11 women and 14 children.
Mattis said it had taken him 30 seconds to decide whether to bomb the location.
Describing the wedding as implausible, he said, "How many people go to the middle of the desert to hold a wedding 80 miles (130 km) from the nearest civilization?
These were more than two dozen military-age males.
Let's not be naive."
The occurrence of a wedding was disputed by military officials, but the Associated Press obtained video footage showing a wedding party and a video the next day showed musical instruments and party decoration among the remains.
When asked by the press about footage on Arabic television of a child's body being lowered into a grave, he replied: "I have not seen the pictures but bad things happen in wars.
I don't have to apologize for the conduct of my men."

Following a Department of Defense survey that showed only 55% of US soldiers and 40% of Marines would report a colleague for abusing civilians, Mattis told Marines in May 2007 that "whenever you show anger or disgust toward civilians, it's a victory for al-Qaeda and other insurgents."
Believing that a need for restraint in war is key to defeating an insurgency, he added: "every time you wave at an Iraqi civilian, al-Qaeda rolls over in its grave."

Mattis popularized the 1st Marine Division's motto "no better friend, no worse enemy", a paraphrase of the epitaph the Roman dictator Lucius Cornelius Sulla wrote for himself, in his open letter to all men within the division for their return to Iraq.
This phrase later became widely publicized during the investigation into the conduct of Lieutenant Ilario Pantano, a platoon commander serving under Mattis.

As his division prepared to ship out, Mattis called in "experts on the Middle East" for "cultural sensitivity training".
He constantly toured the battlefield to tell stories of Marines who were able to show "discretion in moments of high pressure".
As an apparent example, he encouraged his Marines to grow moustaches to look more like the people they were working with.

He also was noted for a willingness to remove senior leaders under his command at a time when the US military seemed unable or unwilling to relieve underperforming or incompetent officers.
During the division's push to Baghdad, Mattis relieved Colonel Joe D. Dowdy, commander of Regimental Combat Team-1.
It was such a rare occurrence in the modern military that it made the front page of newspapers.
Despite this, Mattis declined to comment on the matter publicly other than to say that the practice of officer relief remains alive, or at least "we are doing it in the Marines."
Later interviews of Dowdy's officers and men revealed that "the colonel was doomed partly by an age-old wartime tension: Men versus mission—in which he favored his men", while Mattis insisted on execution of the mission to seize Baghdad swiftly.
As a major general, Mattis commanded the 1st Marine Division during the 2003 invasion of Iraq and the Iraq War. Mattis played key roles in combat operations in Fallujah, including negotiation with the insurgent command inside the city during Operation Vigilant Resolve in April 2004, as well as participation in planning of the subsequent Operation Phantom Fury in November.
In May 2004 Mattis ordered the 3 a.m. bombing of what he alleged was a suspected enemy safe house near the Syrian border, but was in fact a wedding party and resulted in the deaths of 42 civilians, including 11 women and 14 children. Mattis said it had taken him 30 seconds to decide whether to bomb the location. Describing the wedding as implausible, he said, "How many people go to the middle of the desert to hold a wedding 80 miles (130 km) from the nearest civilization? These were more than two dozen military-age males. Let's not be naive." The occurrence of a wedding was disputed by military officials, but the Associated Press obtained video footage showing a wedding party and a video the next day showed musical instruments and party decoration among the remains. When asked by the press about footage on Arabic television of a child's body being lowered into a grave, he replied: "I have not seen the pictures but bad things happen in wars. I don't have to apologize for the conduct of my men."
Following a Department of Defense survey that showed only 55% of US soldiers and 40% of Marines would report a colleague for abusing civilians, Mattis told Marines in May 2007 that "whenever you show anger or disgust toward civilians, it's a victory for al-Qaeda and other insurgents." Believing that a need for restraint in war is key to defeating an insurgency, he added: "every time you wave at an Iraqi civilian, al-Qaeda rolls over in its grave."
Mattis popularized the 1st Marine Division's motto "no better friend, no worse enemy", a paraphrase of the epitaph the Roman dictator Lucius Cornelius Sulla wrote for himself, in his open letter to all men within the division for their return to Iraq. This phrase later became widely publicized during the investigation into the conduct of Lieutenant Ilario Pantano, a platoon commander serving under Mattis.
As his division prepared to ship out, Mattis called in "experts on the Middle East" for "cultural sensitivity training". He constantly toured the battlefield to tell stories of Marines who were able to show "discretion in moments of high pressure". As an apparent example, he encouraged his Marines to grow moustaches to look more like the people they were working with.
He also was noted for a willingness to remove senior leaders under his command at a time when the US military seemed unable or unwilling to relieve underperforming or incompetent officers. During the division's push to Baghdad, Mattis relieved Colonel Joe D. Dowdy, commander of Regimental Combat Team-1. It was such a rare occurrence in the modern military that it made the front page of newspapers. Despite this, Mattis declined to comment on the matter publicly other than to say that the practice of officer relief remains alive, or at least "we are doing it in the Marines." Later interviews of Dowdy's officers and men revealed that "the colonel was doomed partly by an age-old wartime tension: Men versus mission—in which he favored his men", while Mattis insisted on execution of the mission to seize Baghdad swiftly.
Jim Mattis