Whether the only remaining and perhaps highest-profile defendant stands trial may hinge on what happens this week in a military courtroom.

Gen. James Mattis said Monday he never improperly received advice before bringing charges in the biggest criminal case against U.S. troops to arise from the Iraq war.

The four-star general testified for nearly two hours Monday in a pretrial hearing at Camp Pendleton to consider a request by defense attorneys to dismiss charges against Staff Sgt. Frank Wuterich.

Mattis, the former commanding officer for Camp Pendleton testified he was not unlawfully influenced when he brought charges against more than a half-dozen Marines stemming from the so-called Haditha massacre.

On November 19, 2005 in Haditha, Iraq, a squad of Marines used grenades and gunfire to clear a number of homes after a road-side bomb killed a Marine.

In the aftermath of the assault, 24 men women and children lay dead.

The defense argued that Mattis was improperly influenced by Col. John Ewers, who investigated the killings and later became a top legal advisor to the general. Military policy prohibits Ewers from offering legal advice on Haditha because he was also an investigator in the case.

Mattis testified in 2008 that he never talked with Ewers about Haditha, although the aide was at meetings where the case was discussed.

A courtroom appearance is rare for such a high-ranking officer but Mattis has done it before in the Haditha case.

In 2008, a military judge dismissed charges against Lt. Col. Jeffrey Chessani for failing to investigate the Haditha killings after deciding that Mattis had been unduly influenced by the aide.

"The trail basically has already been blazed," Neal Puckett, one of Wuterich's attorneys, said Sunday.

A Marine Corps spokesman, Lt. Col. David Griesmer, declined to comment on the government's case.

Wuterich, 30, is charged with voluntary manslaughter, aggravated assault, reckless endangerment, dereliction of duty and obstruction of justice. He is currently assigned to administrative work at 1st Marine Division headquarters at Camp Pendleton.

Mattis was later promoted to commander of NATO's Supreme Allied Commander Transformation and U.S. Joint Forces Command. He left the NATO post last year but still commands U.S. Joint Forces in Norfolk, Va.

Wuterich's case is before a different judge, Lt. Col. David Jones.

And, while Mattis brought charges in 2006, his successor, Gen. Samuel Helland, court-martialed Wuterich a year later after a preliminary hearing. In Chessani's case, Mattis brought charges and ordered the court-martial.

Wuterich's attorneys said Helland sat in on the same meetings as Mattis.

"(Helland) was probably in the same sort of information flow as Gen. Mattis," said Puckett, who predicts Helland may also testify. "It's going to be the judge's call whether that also taints him."

The deaths occurred after a roadside bomb hit a Marine convoy, killing the driver of a Humvee and wounding two other Marines.

Wuterich and a squad member allegedly shot five men by a car at the scene. Investigators say Wuterich, of Meriden, Conn., then ordered his men to clear several houses with grenades and gunfire, leaving women and children among the dead.

At his preliminary hearing, Wuterich said he regretted the loss of civilian life in Haditha, but said he believed he was operating within military combat rules when he ordered his men to attack.

The pretrial hearing is scheduled to last up to one week.

Puckett said the judge has told attorneys he expected to rule by the end of this week. If the defense argument fails, attorneys for both sides have agreed to go to trial in September, he said.