A decision by a military judge on Friday to disqualify a top Pentagon official from any further role in a Guantánamo war crimes case was a major new challenge to the Bush administration’s legal approach to the war on terrorism.

The ruling, in the case against Salim Hamdan, a detainee who was a driver for Osama bin Laden, transformed what had been something of a Pentagon soap opera over how to prosecute detainees into a formal ruling that gave new force to critics’ accusations of improper political influence over this country’s first use of military commissions since World War II.

At issue is the role of a Pentagon office called the “convening authority,” which oversees the military prosecutors and has extensive power over the defense lawyers and judges in the cases against Guantánamo detainees. One role of that office is to be a neutral arbiter, deciding such matters as allocation of resources for both the defense and prosecution and which charges brought by prosecutors should go to trial.

But military defense lawyers and other critics have said officials running that office have overstepped the bounds of impartiality by pushing prosecutors to charge more detainees and to use evidence obtained under coercive interrogations.

Lawyers said the ruling set the stage for new challenges that could slow even the administration’s highest priority Guantánamo prosecution, against six detainees for the 2001 terrorist attacks. One of the six is Khalid Shaikh Mohammed, the self-professed planner of the attacks that killed nearly 3,000 people.

“The military judge has said that, at the very least, there are grave appearance problems with this system,” said Michael J. Berrigan, the deputy chief defense counsel for the Guantánamo cases.

A kangaroo court or kangaroo trial, sometimes likened to a drumhead court-martial or Drumhead trial, is a sham legal proceeding or court. Kangaroo courts are judicial proceedings that deny due process in the name of expediency. The outcome of such a trial is essentially made in advance, usually for the purpose of providing a conviction, either by going through the motions of manipulated procedure or by allowing no defense at all.

The term is often applied to courts subjectively judged as such, while others consider the court to be legitimate and legal. A kangaroo court may be a court that has had its integrity compromised; for example, if the judge is not impartial and refuses to be recused.

It may also be an elaborately scripted event intended to appear fair while having the outcome predetermined from the start. Terms meaning "show trial", like the German Schauprozess, indicate the result is fixed before (usually guilty): the "trial" is just for show. Notorious were Soviet dictator Joseph Stalin's kangaroo trials against his enemies, whom he labeled enemies of the people, notably in the context of the Great Purge. Another example is Roland Freisler's "processes" against the enemies of the National-Socialist regime.