What is the Point of a Military Trial?

I mean the title of my post today quite literally. The U.S. has charged Canadian Gitmo detainee Omar Khadr with murder in connection with his having thrown a grenade that killed Army Sergeant Christopher Speer during fighting in Afghanistan. Khadr was allegedly part of al Qaeda.

Throwing a grenade at soldiers looks more like warfare and less like terrorism than most of the things that al Qaeda members have done, even if it constitutes a violation of the law of war because al Qaeda forces don't abide by the laws of war. (For example, they don't wear distinctive uniforms, thus permitting them to blend in with, and thereby endanger, the civilian population.) But that still leaves the question of what advantage the U.S. gets from trying the likes of Khadr, rather than just continuing to detain him as an enemy combatant.

One possibility is that the government fears an eventual Supreme Court ruling that enemy combatants cannot be held forever. If so, there would need to be some reason other than simple incapacitation to continue holding them. But there are two difficulties with this theory. First, it's not at all clear that the Supreme Court would place time limits on the holding of enemy combatants, especially after the Military Commissions Act of 2006. Second, the government does not appear to be pursuing military trials as a means of holding detainees longer than it otherwise could. For example, Australian David Hicks got a plea deal in which he will only serve 9 months.

But part of the Hicks deal is his agreement not to make public statements about how he was treated while a detainee. Accordingly, one might think that the point of these trials is to buy detainees' silence about mistreatment. That idea has been floated but it too makes little sense. A much more effective way to keep Hicks quiet would have been simply to keep holding him.

Which brings me back to my genuine puzzlement. Perhaps the Hicks case was an aberration, and the government plans to seek life sentences for Khadr and others. If not, there is a real tactical mystery here.