Call it the triumph of ideology over national interest and honor. Having dithered for nearly three years, the Obama administration has only a few weeks to bring to justice a Hezbollah terrorist who slaughtered five U.S. soldiers in Iraq in 2007. Unfortunately, it appears more likely that Ali Musa Daqduq will instead be transferred to Iran, to a hero's welcome.

In the early evening of Jan. 20, 2007, in the city of Karbala, south of Baghdad, five black SUVs approached the location of a regular meeting between U.S. and Iraqi military officers. Inside the vehicles, which mimicked U.S. transports (to avoid heightened scrutiny), were a dozen individuals dressed in U.S. military uniforms and bearing U.S. weapons. Their drivers spoke English.

Upon reaching their target, the occupants opened fire on the Americans. One U.S. soldier was killed on the spot. Four others were kidnapped, tortured and executed.

The mastermind of this brutal attack? Ali Musa Daqduq, a Lebanese national and Hezbollah commander. U.S. forces captured him in March 2007, and, in interrogation, he allegedly provided a wealth of information on Iran's role in fomenting, training and arming Iraqi insurgents of all stripes.

With U.S. troops set to exit Iraq at the end of December, all detainees in American custody there have been transferred to the Iraqis except for Daqduq. He is set to be turned over in a matter of weeks. Based on past experience with released detainees who were in Iranian employ, U.S. officials know that Daqduq will promptly re-emerge in Iran, shaking hands with dignitaries and leading parades, before rejoining his Hezbollah colleagues.

This outcome would be an insult to the American servicemen who have lost many comrades to insurgents such as Daqduq, who consistently failed to comply with the laws of war. Indeed, the Iraq war is the first conflict in modern history where the U.S.—having complied with the laws of war by promptly prosecuting American troops believed to have violated those laws—did not bring to justice a single one of the hundreds of captured enemy combatants who have killed Iraqi civilians, American soldiers and contractors. Impunity for war criminals debases the laws of war, violates our international legal obligations, and is inconsistent with American values.

We have already failed to stop Iran's nuclear-weapons program. We have also failed to punish Tehran for facilitating the deaths of American soldiers, or for plotting to assassinate the Saudi ambassador to Washington. Allowing Daqduq to slip through U.S. hands would further reinforce the impression of American impotence. That will have serious repercussions, measured in diplomatic defeats and lost lives.

There is an obvious solution: Transfer Daqduq from Iraq to Guantanamo Bay to be tried by a military commission there. But this is where the Obama administration's rigid ideology comes into play—beginning with flawed, self-defeating legalistic arguments.

A successful prosecution of Daqduq would be relatively easy. He killed American soldiers and, as an unprivileged belligerent, has no combatant immunity. Yet the administration purports to be troubled by our lack of an extradition treaty with Iraq. It also points out that the Iraqis have refused to accord the U.S. legal custody of Daqduq, although the U.S. has him in physical custody. The Iraqis, of course, are being pressured by the Iranians not to accommodate this legal-custody request.

Yet we don't need an extradition treaty with Iraq to transfer Daqduq, a Lebanese citizen captured by American forces in a war zone. Since his capture occurred when the U.S. and other coalition members were the occupying power in Iraq, there is ample basis in existing international law for the American exercise of legal jurisdiction over him.

A more serious obstacle is the administration's policy of eschewing military tribunals. Earlier this year, the administration considered bringing Daqduq into the U.S. to face trial in a civilian court. In response, six Republican senators wrote President Obama, warning against trying Daqduq in federal court, and urging the president to refer him to a military commission.

The administration briefly flirted with the idea of a military commission, perhaps in Charleston, S.C. or at Fort Leavenworth, Kan. That idea seems to have been dropped after a Nov. 8 Senate Judiciary Committee hearing where Republican Sen. Lindsey Graham told Attorney General Eric Holder that if the administration were to bring Daqduq to the U.S. for a civilian or military trial, "all hell would break loose."

The administration believes that bringing anyone new, even high-value detainees, to Guantanamo is inconsistent with the goal of eventually closing the facility. This proposition is absurd, and not only because that facility remains vital and relevant to this day. It raises the question of whether administration's detention policy is actually shaped by a crass political calculus of not antagonizing its liberal base in advance of what promises to be a difficult 2012 election.

The administration should press the Maliki government in Baghdad harder to allow the U.S. to maintain custody of Daqduq following the withdrawal of U.S. forces from Iraq. If the Iraqis still refuse, the administration should unilaterally transfer Daqduq to Guantanamo to face justice.

While the Maliki government may protest publicly, it will rejoice privately, since Daqduq's rendition would demonstrate Washington's resolve in the face of Tehran's pressure. Allowing him to go unpunished is both inexcusable and dangerous.

Mr. Rivkin served in the Justice Department during the Reagan and George H.W. Bush administrations. Mr. Stimson, senior legal fellow at the Heritage Foundation, was a deputy assistant secretary for detainee affairs at the Defense Department.