The suit, brought by 19 Iraqis, accuses General Franks of war crimes during the Iraq conflict.
Lawyer Jan Fermon, who is acting on behalf of the Iraqis, described the plaintiffs as victims of cluster bombs and of US attacks on ambulances and civilians.
"We have a very specific case, with specific evidence," Mr Fermon said. "I do not see how they can reject it."
The case would be presented in court on 13 May, he said, including evidence which had been gathered by Belgian doctors working in Baghdad.
Mr Fermon said there were 17 "specific incidents" in which US soldiers and commanders had violated the law.
Belgium's war crimes legislation, allowing suits involving cases which happened abroad, has proved highly contentious since its introduction in 1993.
It has recently been softened to make it harder for cases to proceed.