When Congress passed the Military Commissions Act in October 2006, one major concern was that the intended targets of the statute - 9/11 plotters and Al Qaeda members recently transferred to Guantánamo from CIA black sites - were expected to be tried for things they had done well prior to the passage of the Act.

The Constitution generally prohibits retrospective application of new criminal laws, so everyone assumed the only way the new statute could be used to punish past acts was if the Military Commissions Act merely codified crimes that had always been understood to be offenses against the law of war, triable by military commissions.

As it turned out, the most popular charges for commission prosecutors are those that don't necessarily involve a completed offense - conspiracy (the crime of planning a crime) and material support (the crime of association with terrorist organizations). The crime of "material support" is a relatively modern innovation: it wasn't even a civilian federal crime until 1996, and conspiracy had only an isolated history of being used in American military commissions, the charge being largely unknown to the legal systems of much of the world. In our rough count, 29 of 32 military commission charge sheets have involved at least one of these charges.

So it was with much anticipation that the Court of Appeals for the D.C. Circuit decided an appeal by a detainee, alleged al Qaeda propagandist and Bin Laden confidante Ali al Bahlul, that challenged whether conspiracy and material support were in fact traditionally held to be crimes under the law of war. Because of the importance of the issue to the future of the commissions system, the full court (rather than a three-judge panel) heard the case last September --a rare procedure reserved for a handful of cases a year.

Nine and a half months passed before the court issued its opinion this Monday. After all that time, and with so much at stake, what did the court actually decide? As it turns out: next to nothing.

Does the constitutional prohibition against retroactive criminal laws even apply at Guantánamo? The court didn't answer that question. Instead it merely presumed that there was such a prohibition, simply because the Justice Department conceded the point in its arguments. The concession forced the court to address the big issue: whether conspiracy was an established offense under the customary law of war - the baseline of acts broadly acknowledged by the international community to be war crimes - or whether it is enough to simply find that conspiracy was established as a war crime under the "domestic common law" of war - the past practices of only American military commissions, or other purely domestic sources that show that an act was already a crime triable by military commission.

Again, despite the rare convening of the full court, and 150 pages of opinions, the court gave no answer. Instead, the court held that because Bahlul (who effectively represented himself) didn't put on any real defense at trial, he forfeited all these claims. On appeal in such situations, federal courts only look to see whether there was error so plain that it "seriously affects the fairness, integrity or public reputation" of the proceedings. In other words, the court didn't have to decide whether DOJ's theories for upholding the conviction were right; it merely had to decide whether it was embarrassingly obvious that they were wrong. The D.C. Circuit reasoned that since it wasn't egregiously obvious that the government had made a mistake, Bahlul's conviction should be upheld. But a decision on whether DOJ's arguments were actually right (rather than simply "not laughably wrong") will have to wait for some future case.

Of course, that's a pyrrhic victory for the Justice Department. The very limited ruling offers absolutely no clarity on these big questions that will come up eventually in every future military commission case - with, perhaps, one exception. All seven judges agreed that "material support for terrorism" is not even arguably a war crime triable by military commission, and vacated Bahlul's conviction on that one count. That is great news for Australian David Hicks, who when he pled guilty to a single "material support" charge back in 2007 became the very first person convicted under the Military Commissions Act. His appeal was held pending the Bahlul decision, and in the wake of that decision one would be hard pressed to understand why the government would even bother continuing to argue that it should stand. Monday's ruling, then, is a reminder that a military commission prosecution or conviction can self-destruct at any time.

A year and a half ago, one of us wrote that the government's appeal was a delaying tactic designed to stave off the day when the administration had to make the inevitable decision to scrap the military commission system entirely and bring cases to trial in the domestic federal criminal courts, no matter how politically charged Congress has made that prospect. The New York Times reported in January 2013 that the head of the commissions, President Obama's law school friend and one-star general Mark Martins, thinks the Justice Department's arguments in Bahlul were bunk, and that the commissions shouldn't seek to use either material support or conspiracy as charges. The Justice Department apparently only wanted to defend the law because Congress had passed it.

Outside Congress, it seems that no one believes that the military commission system should continue to stand. For twelve years, the system has created nothing but embarrassment for its proponents as the highest-profile prosecutions have repeatedly tripped over their own feet. The first cases have also created waves of fundamental legal challenges. This week's decision simply postponed the most basic issues to be resolved by some future case. That leaves a very real prospect that the 9/11 prosecutions fall apart up on appeal years after trial court proceedings are over, as Bahlul's material support conviction did. The administration wants to try terrorism suspects in federal court for good reason. Congress should not stand in its way.

Hu and Kadidal are attorneys at the Center for Constitutional Rights in New York and represent David Hicks and numerous other former and current Guantánamo detainees.