GUANTÁNAMO BAY, Cuba  A panel of six military officers convicted a former driver for Osama bin Laden of one war crime Wednesday but acquitted him of another, completing the first military commission trial here and the first conducted by the United States since the end of World War II.

In a stinging setback for the military prosecutors, the commission acquitted the former driver, Salim Ahmed Hamdan, of a conspiracy charge, arguably the more serious of two charges he faced. Mr. Hamdan’s conviction came on a separate charge of providing material support for terrorism.

The split verdict gave both sides in the long debate over the procedures here grounds for their competing claims. Supporters said that the system’s fairness was illustrated by the careful verdict while critics said the trial, which featured secret evidence and closed proceedings, demonstrated the injustice of the Bush administration’s military commission system.

Mr. Hamdan, who has said he is about 40, faces a possible life term. The sentence is to be set in a separate proceeding before the same panel that is to begin this afternoon. As the verdict was read, Mr. Hamdan, who has been in custody since he was detained in Afghanistan in November of 2001, stood passively at the defense table in a white headscarf, his head bent slightly down.