"Drumhead trial" means the same thing, musick:

A kangaroo court (also known as a kangaroo trial or a Drumhead trial) is a sham legal proceeding or court, often judged subjectively through the speaker's perspective. It may be a court that has had its integrity compromised, i.e., the impartiality of the judge who refuses to be recused, or an elaborately scripted event intended to appear fair while having the outcome predetermined from the start.

One assumes a "drumhead trial" is a hasty, 'let him plead and then shoot him' caricature of a trial, with a roll of drums preceding the shooting for ceremonial effect.

No doubt, justice was speedy and predetermined to allow the judge and jury to return to more serious business killing the enemy.