Conservative infighting over whether the United States should intervene militarily in Libya is sign of good health. To remain vital, an ideological movement needs to have its basic assumptions challenged occasionally. We need to correct our wayward courses rather than allow mistakes based on faulty strategy to serve as precedents for the next missteps.

Many conservatives (particularly neoconservatives) are strong supporters of intervention, out of a deep conviction that the global advance of freedom promotes American security. I happen to disagree, at least insofar as the “freedom agenda” relies on the U.S. military as its agent. Regardless of where one comes out on the policy, though, we all ought to agree on at least one thing: The Constitution must control the implementation of whatever policy wins the day. Yet it has become necessary to ask whether even this principle, so fundamental to a free, self-determining people, is still unanimously honored.

On Thursday evening, the U.N. Security Council voted 10–0 (with five abstentions, including China, Russia, and Germany) to authorize the use of military force (i.e., “all necessary measures”) against Libya. Ostensibly, the resolution is designed to protect the Libyan people. But not to mince words, it is a license for war against the regime of Moammar Qaddafi. It would kick hostilities off with a no-fly zone over Libya. As a practical matter, American armed forces must do the heavy lifting if the strategy is to have a prayer, and indications are that President Obama intends to oblige.

There is a catch: The Security Council is powerless to “authorize” the U.S. military to do a damned thing. The validity of American combat operations is a matter of American law, and that means Congress must authorize them.

Our Constitution vests Congress with the power to declare war. That authority cannot be delegated to an international tribunal that lacks political accountability to the American people. The decision to go to war is the most significant one a body politic can make. Thus the Framers designed our system to make certain that the responsible officials are answerable to the people whose lives are at stake and who are expected to foot the bills.

Contrary to the insistence of many on the right and the left, this has never meant that military operations may not be launched in the absence of a declaration of war. Indeed, although the United States has engaged in many wars and lesser conflicts, war has been formally declared only five times. Still, the circumstances for departing from this formality are narrowly defined.

The first is obvious: an actual or imminent strike against the United States. There is no question that the government’s principal responsibility is the security of the governed. The president, as commander-in-chief, has not only the authority but the duty to order the use of any force necessary to protect the United States from hostile powers that attack or are preparing to attack. The principle is clear, and the Supreme Court has endorsed it since the 19th century.

But this is not a blank check for the president. At a certain point in time — which may vary with the peculiar circumstances of different armed conflicts — Congress must weigh in and either endorse or put a stop to presidential war-making. It may do the latter by refusing to support the president’s actions and calling for military operations to cease. If the president refuses to honor this expression of disfavor by the people’s representatives, Congress may use its power of the purse to defund military operations. If the president tries to persist, Congress may even impeach him. The central point is that the commander-in-chief’s brute power is not a limitless authority. To be valid, combat operations must at some early, practical point be blessed by Congress, even if the president has righteously ordered them in response to an attack on our country.

The other circumstance of departure from the formal declaration of war occurs when Congress functionally declares war by approving combat without using the magic words “declare war.” A good example of this is the authorization for the use of military force Congress enacted, and President Bush signed, in the days after 9/11. Some on the left — largely to bolster their contention that terrorism should be treated as a law-enforcement issue — take the position that sovereign nations cannot truly engage in war with sub-sovereign terrorist organizations. Furthermore, it was unknown right after 9/11 (in fact, it remains unknown today) whether any foreign nation knowingly assisted al-Qaeda in the 9/11 plot. To avoid bogging down over such matters, Congress substantially declared war without using those words: It provided the president with sweeping authority to attack the enemy and reaffirmed that authorization several times with funding.

Note that both situations warranting departure from the formal declaration of war involve actual danger to the United States (or, worse, completed attacks against the United States). That is common sense. Government cannot perform its essential security function unless the president can act in a true crisis. The nation could otherwise be destroyed. In due course, though, Congress must endorse or reject presidential action. The system is designed to ensure that the nation can protect itself but does not commit to war unless the American people consent.