When President Obama relinquished the term “enemy combatant” from the government’s lexicon as a justification for holding prisoners indefinitely without charge or trial, he didn’t give up the power to hold people he deemed were fighting the United States. But the question remained: how does the government decide who those people are, and what evidence does it have to present to justify its determination?

That’s been left to the federal district courts hearing the habeas corpus cases of Guantanamo detainees’ to decide, and yesterday, Judge John Bates, a Bush appointee on the federal court in Washington, D.C., significantly narrowed the definition of who can legitimately be deemed a combatant (pdf). He did not, however, reject the government’s claim that under the laws of war, it can hold enemy fighters indefinitely — much to the dismay of some of the detainees’ defense lawyers.

The Obama administration had claimed “the authority to detain persons that the President determines planned, authorized, committed, or aided the terrorist attacks that occurred on September 11, 2001″; “persons who harbored those responsible for those attacks; “and “persons who were part of, or substantially supported, Taliban or al Qaida forces or associated forces that are engaged in hostilities against the United States or its coalition partners, including any person who has committed a belligerent act, or has directly supported hostilities, in aid of such enemy armed forces.”

Bates, while accepting the still-controversial view that the laws of war allow the president to detain al-Qaeda or Taliban fighters indefinitely, refused to extend that to those who “support” those groups, saying “the Court can find no authority in domestic law or the law of war, nor can the government point to any, to justify the concept of “support” as a valid ground for detention.”

However, citing Congress’s declaration in the Authorization for the Use of Military Force (AUMF) against organizations that the president determines attacked the United States on September 11, 2001 and analogizing from the laws of war, Bates concluded that the government can detain members of al-Qaeda and the Taliban, and members of “associated forces” or “co-belligerents.”

But how do you prove who was a “member” of al-Qaeda or the Taliban? Members of terrorist organizations don’t wear uniforms or carry cards declaring their membership. And that’s, of course, what the government and defense lawyers will be fighting out over the coming months.

Already, in several cases — such as in the Alla Ali Bin Ali Ahmed case that I wrote about here — judges have determined that the government did not have enough evidence to support its “membership” claim. Staying in a guest house frequented by al-Qaeda members, for example, was not enough to prove membership. And tending to al-Qaeda members as a doctor or cleric, Bates pointed out in his opinion yesterday, would not alone be enough.

Defense lawyers representing Gitmo detainees say that in many of their cases, that’s exactly the sort of flimsy evidence the government is claiming justifies the detainees’ indefinite detention until the end of the “war on terror.” Bates yesterday brought those detainees a step closer to their eventual release.