Sergeant First Class Tracy Perkins faces up to 29 years of confinement on charges of involuntary manslaughter, aggravated assault, conspiracy and obstruction of justice.
Witnesses called on Tuesday by the prosecution said they heard Perkins say over the radio: "Somebody is going to get wet tonight."
But one of Perkins' lawyers, Captain Thomas Hurley, said prosecutors could not even prove a homicide, much less a criminal case.
"There was no death," Hurley said in opening statements.
The trial is expected to run for three days.
A number of US soldiers have
been put on trial for abusing Iraqis
Prosecutors said a patrol in a platoon led by Perkins forced 19-year-old Zaidun Hassun and another man off a bridge and that Hassun drowned.
"Zaidun was found, face-down, dead," said prosecutor Captain Tom Schiffer.
The two Iraqis were suspected of violating a curfew on 3 January 2004, near Samarra, 100km north of Baghdad.
While prosecutors and defence lawyers differed on whether Hassun drowned, both sides agreed that the other Iraqi man survived the encounter with the patrol.
Schiffer's first witness, army crime scene investigator Irene Cintron, said the surviving Iraqi told her the two men were pushed and fell three to four metres.
However, Cintron acknowledged on cross-examination that no
body was recovered or exhumed after Hassun's family said it
had buried him.
She said she did not have a pathologist review a videotape of what the family said was Hassun's body.
Cintron also told the panel - the military equivalent of a jury - that she never measured the water's depth and observed there was "not much of a current".
"Somebody is going to get wet tonight"
Witnesses quoting Tracy Perkins
The panel in the court martial consists of three officers and three enlisted personnel.
Members of the army patrol that forced the Iraqis into the river said the pair were taken to the water's edge at gunpoint.
The two entered the water a few seconds apart and one of the two was soon standing on a nearby river bank while Hassun was still in water that appeared up to his shoulders when they left the scene.
Sergeant Alexis Rincon, who was in the group that forced the men off the bridge, said the orders to push the men in the river were given by 1st Lieutenant Jack Saville and implemented by Sergeant Reggie Martinez.
Both have been charged in the incident.
Perkins was in a different vehicle stopped about 100m from the scene and did not exit the vehicle, several soldiers testified.