The news that the youngest of the Guantánamo detainees, Omar Khadr, has been sentenced to eight years in a maximum security prison for the killing of US special forces medic Christopher Speer is a reminder that some of the more contentious practices in America's war on terror continue. The charges against Khadr reflect US attempts to rewrite international law – and the implications for the laws of war are potentially profound.

Khadr's case was heard by a military commission rather than a federal court, and rights groups have consistently denounced the procedures leading up to the plea bargain. Of course, President Obama promised to close Guantánamo Bay within a year of taking office. Now, at the mid-point of his first term, 174 detainees remain in the camp, and the plan to close it either by repatriating them, prosecuting them in federal courts, or transferring them to a "supermax" prison on the mainland, has stalled because of political opposition.

The Obama administration justifies the use of military commissions and what it calls "prolonged detention" by referring to the president's war powers, which, it reminds us, were delegated by Congress when it passed the Authorisation to Use Military Force (AUMF) on 18 September 2001. It uses the same argument to justify the equally contentious practice of "targeted killing".

Liberals continue to argue that such actions are illegal outside the battlefield, or that they can only be justified in circumstances when there is no other way of preventing a terrorist atrocity. If there is one single, compelling piece of evidence that the post 9/11 American exception has outlasted the Bush administration, then its successor's continued reliance on the AUMF is it.

Much of the controversy surrounding Khadr related to his age at the time he was detained on the Afghan battlefield: he was 15. Given his juvenile status, at the time he threw the grenade that killed Speer, he should have been subject to a different judicial process. There are other aspects of the case that attract attention, including the Canadian government's refusal to request extradition and its role, now apparent in facilitating the plea bargain, as well as what should have been the admissibility of a confession allegedly extracted through torture.

What has not been reported so widely, however, is the nature of the charges against Khadr and the impact his guilty plea could have on the laws of war.

The issue here is that "murder in violation of the laws of war" – the charge laid against Khadr – is not recognised internationally as a war crime. Following the Bush administration's lead, the US Congress insisted in the 2009 Military Commission Act that any civilian targeting an American soldier was an unprivileged belligerent guilty of murder or attempted murder. But "unprivileged belligerent" is not a category of combatant defined in the laws of war.

Under the Geneva conventions, violent individuals are either combatants or civilians. As an enemy combatant, one would have expected Khadr (age aside) to be targeting US soldiers. The only way the US government could have prosecuted him under the laws of war, therefore, was to charge him with killing while disguised as a civilian or with "perfidy".

As the defence counsel argued in the similar case of Mohammed Jawad, however, al-Qaida fighters do not have uniforms to give up, so their civilian appearance was not a disguise. They were, therefore, in the eyes of the laws of war, civilians. As a non-combatant throwing a grenade, Khadr could certainly have been prosecuted for murder – but not under the laws of war and not in a military commission. He should have been due a trial in the civilian courts of Afghanistan, the US or Canada, where the rules of evidence would have been very different.

The implication of finding a civilian guilty of murder in violation of the laws of war is significant. It is another instance where the US has "militarised" civilian crimes. Other detainees, for instance, have been found guilty by military commission for providing material support for terrorism, and rights groups continue to fear that the US interpretation of "enemy combatant" is now so broad that a proverbial "little old lady in Switzerland" giving to a charity with armed Islamist connections could be tried in a military commission.

Above all, the Khadr ruling is a case where the US has pursued its own definition of the laws of war. "Murder in violation of the laws of war" was codified by the US Congress alone, and many experts consider it a rank misinterpretation of international law. If other states adopt the American example, then we may look back at the Khadr trial as the moment when a new form of war crime was created, unilaterally and for the expediency of the world's pre-eminent military power.