NEWPORT BEACH, Calif. — Stephen E. Abraham’s assignment to the Pentagon unit that runs the hearings at Guantánamo Bay, Cuba, seemed a perfect fit.

A lawyer in civilian life, he had been decorated for counterespionage and counterterrorism work during 22 years as a reserve Army intelligence officer in which he rose to the rank of lieutenant colonel. His posting, just as the Guantánamo hearings were accelerating in 2004, gave him a close-up view of the government’s detention policies.

It also turned him into one of the Bush administration’s most unlikely adversaries.

In June, Colonel Abraham became the first military insider to criticize publicly the Guantánamo hearings, which determine whether detainees should be held indefinitely as enemy combatants. Just days after detainees’ lawyers submitted an affidavit containing his criticisms, the United States Supreme Court reversed itself and agreed to hear an appeal arguing that the hearings are unjust and that detainees have a right to contest their detentions in federal court.

Some lawyers say Colonel Abraham’s account — of a hearing procedure that he described as deeply flawed and largely a tool for commanders to rubber-stamp decisions they had already made — may have played an important role in the justices’ highly unusual reversal. That decision once again brought the administration face to face with the vexing legal, political and diplomatic questions about the fate of Guantánamo and the roughly 360 men still held there.