Watch Only the Brave (1930) Official Trailer

Cooper is a Union Army officer who after being jilted by girlfriend, Virginia Bruce, volunteers on what could be a suicide mission. He volunteers to go behind enemy lines disguised in Confederate gray as a staff officer to Robert E. Lee. He's to ride to a certain plantation which is a local brigade headquarters and deliberately let himself be caught with maps showing false Union troop positions. Of course, the penalty, then as now, is execution.