The Obama administration waited until 5:45 p.m. on a Friday in late June to float the idea that it is considering an executive order authorizing prolonged detention of captured enemy combatants.

The announcement was timed to run when most Washingtonians were tippling cocktails or en route to the beach to avoid expending much political capital on the issue. This approach won't work.

The chief problem is that in the course of shutting down the detention facility at Guantanamo Bay, President Obama has discovered, just like President Bush before him, that there are no easy answers to the question of what to do with detainees suspected of being highly trained terrorists.

For those detainees who cannot be safely tried or transferred to third countries, the best option, probably the only option, is a military detention system (or, as Mr. Obama calls it, "prolonged detention").

Like Mr. Bush before him, Mr. Obama has the instinct to go it alone, working by executive order and without any input from Congress. Mr. Obama's advisers, The Washington Post reports, are concerned that pursuing legislation could give "Congress too much control over the process."

They have good reason to be concerned, given Congress' unwillingness to confront the issue head-on. Congressional Democrats, in particular, have been long on criticism but short on solutions. They have rejected release, incarceration in the United States and seemingly any kind of long-term detention policy, whether centered in Guantanamo or anywhere else.

What the Democrats need, and what the president has yet to give them, is a refresher course on the law of war. The Geneva Conventions and customary laws of war authorize the detention of the enemy during wartime. The United States is at war; both Mr. Bush and Mr. Obama agree on that fact. Some of our enemies are detained at Guantanamo Bay and need to be held for the foreseeable future while we prosecute this war against al Qaeda, the Taliban and their allies.

Contrary to the claims of the American Civil Liberties Union and other activist groups, there is no Legal requirement to try these enemies or set them free. That's a criminal law concept, and it is not part of the law of war. Mr. Obama now understands this, but will his former colleagues in Congress?

Recall that Democrats beat up the Bush administration for years about its detention policies, its "go it alone" mentality and its purported failure to adhere to the Geneva Conventions. Will those positions flip now that the White House has changed hands? So far, it hasn't.

Instead, the Democrats block funds to close Guantanamo or transfer any detainees to the United States. They demand that the president give them advance notice of any impending transfers to the United States for criminal prosecution. Of course, some of these members of Congress are the same ones who, during the Bush administration, claimed that the detainees at Guantanamo weren't dangerous and that we had all the wrong people there, anyway.

So the president is probably right to recognize that legislating military detention would be a heavy lift.

But that doesn't mean he has a choice in the matter. After Rasul, Hamdi, Hamdan and last year's Boumediene, it's clear that the Supreme Court is wary of the war on terrorism and especially wary of detainee policy by order of the executive.

There's no reason to believe the court would ultimately uphold any plan hatched by the Obama administration alone, short of immediate criminal trial or immediate release.

The president, of course, is well aware of this. The cynical case for detention by executive order is that it will let the president demonstrate leadership on the issue while giving him political cover for the eventual release or resettlement in the United States of all the terrorist detainees. It buys him some time, as the cases wind through the courts, before that happens.

But kicking the can down the road may not work this time if the courts decide enough is enough. That could happen fast, and it wouldn't leave the president many options.

The only viable option then would be for Mr. Obama to go all in on this issue. He should challenge Congress to devise a system of military detention for those few captured fighters in the war on terrorism who cannot be prosecuted or let go. This will take political capital, lots of it, but there's no other choice if the president takes seriously his duty to protect the American people.

David B. Rivkin Jr. is a partner in the D.C. law offices of Baker Hostetler LLP and a co-chairman of the Center for Law and Counterterrorism at the Foundation for Defense of Democracies. Charles "Cully" D. Stimson is a Senior Legal Fellow at the Heritage Foundation, and a former Deputy Assistant Secretary of Defense for Detainee Affairs in 2006-2007.