The last “declared war” the United States fought was against Panama in 1989-90. (America didn’t declare it; the Panamanian General Assembly did. It didn’t work out well for them.)

To many Americans, the phrase “declare war” has a kind of magic meaning. Congress has the power under Article I, Section 8 of the Constitution to “declare war, grant letters of marque and reprisal, and make rules concerning captures on land and water.” Thus, many imagine that, to be constitutional, a military operation must be preceded by a “declaration of war.” Take, for example, Senator Rand Paul, who late last year introduced a resolution that would have declared a “state of war” between the U.S. and the Islamic State; or columnist Charles Krauthammer, who on February 4 said that “the idea I think is to declare war as we used to do 70 years ago and say ISIS, Islamic State, and that means against a certain entity anywhere it is ... Why not do it that way then revive the constitutional way of doing it?” Some commentators—like syndicated columnist Walter Rodgers—go so far as to insist that every use of force by the U.S. since World War II has been unconstitutional, even if authorized by Congress, because the authorizations did not contain the magic words “declare war.”

But law isn’t magic. As Saikrishna Bangalore Prakash of the University of Virginia wrote in 2008, from the very beginning of the Republic it has been understood that Congress could authorize war without the magic words. “[T]he declare-war power matters not because Congress must formally declare war prior to the nation waging war,” he wrote, “but rather because the grant of the declare-war power was premised on the more fundamental proposition that Congress must decide whether the nation will wage war.”

That is the question the U.S. should be debating: Shall it fight ISIS—and if so, how? Congress is just beginning to grapple with President Obama’s request for authority to attack the Islamic State. What they do in the next few weeks will powerfully shape history over the next decade. The issue is not whether the words “declare war” are used; the issue is what the resolution authorizes and does not.

As Prakash noted, the founding generation never imagined that Congress could not authorize military force without using the magic words “declare war.” In fact, in 1788, Alexander Hamilton noted in Federalist 25 that “the ceremony of a formal denunciation of war has of late fallen into disuse.” From the “Quasi War” with France in 1798 through the battles against the Barbary Pirates under Jefferson and Monroe, Congress used different forms of words to authorize presidents to use force—and to limit the kind of force that could be used. In 1798, for example, the Act Further to Protect the Commerce of the United States authorized President John Adams to use U.S. naval forces, and privateers, to seize “armed French vessels” on the high seas—but withheld authorization to use ground forces against the French enemy. In 1815, President James Madison asked Congress to proclaim “a state of war” between the U.S. and the Dey of Algiers; Congress refused. Instead, it authorized him to use the Navy to protect U.S. shipping and seize Algerian vessels.

“Declarations” were intended to signal the existence of a “state of war” between two nation-states. This legal concept triggered certain rights and responsibilities between the warring powers. But there were reasons why states would not want to recognize that “state of war”—it signaled implicit recognition of the enemy as a nation. In 1861, Abraham Lincoln blockaded Southern ports. Instead, Lincoln disdainfully referred to the CSA as a mob: "combinations too powerful to be suppressed by the ordinary course of judicial proceedings." To have “declared war” might imply recognizing the Confederacy as a sovereign state. It would be a great propaganda victory for ISIS if the U.S. “declared war” on them.

International law scholar Malcolm N. Shaw defined “war” as “a legal state of affairs which permitted force to be used and in which a series of regulatory conditions were recognized.” But, he added, “It is no longer possible to set up the legal relationship of war in international society.” The Kellogg-Briand Pact of 1928 committed most of the world, including the U.S., to “condemn recourse to war for the solution of international controversies, and renounce it, as an instrument of national policy in their relations with one another.” It’s easy now to laugh at the pact; but in 1945, Nazi violations of it were one legal base for the Nuremberg Trials, and it remains in effect. That same year, the victorious Allies created the United Nations Charter. U.N. members have renounced “the threat or use of force against the territorial integrity or political independence of any state” except when authorized by the Security Council or in self-defense.

In other words, the “state of war” is as obsolete as the letter of marque.

The idea of “declaring war” has a nostalgic, even comforting, ring, though. The U.S. last did it in World War II, when the nation was united and the only aim was “unconditional surrender.” That’s the idea lurking behind Senator Marco Rubio’s perfectly terrible suggestion that a new authorization consist of “one sentence: ‘We authorize the president to defeat and destroy ISIL.’ Period.”

But most wars, before or since, have not been total ones. As noted above, as early as 1798, Congress was using its war power to instruct the commander-in-chief what forces he could use and which ones he could not. Members of Congress should be as fearless in discussing limitations as they are in denouncing ISIS. This moment—when an administration has requested congressional approval for a military campaign—is probably the only real opportunity Congress and the people will ever have to shape what happens after the vote. Limits of time, place, and means can be written into an authorization now, requiring the president to agree to them or veto the bill. As America learned in Vietnam, once an authorization is passed, it is virtually impossible to change.

The Obama administration has taken a remarkable step forward by proposing an authorization with a time limit. Their proposed resolution, however, has no geographic limits, and it allows the executive broad latitude to widen the war against “individuals and organizations fighting for, on behalf of, or alongside ISIL or any closely-related successor entity.” The restriction on “enduring offensive ground combat operations” means almost nothing, because there simply is no legal definition of “enduring”—and its connotation is ominously long.   

The war in Vietnam was supposed to be a short intervention; no commander ever announces, “today, we enter a quagmire”? Obama's draft also retains in force the authorization against al-Qaeda passed in 2001—which is so loosely written that Obama has claimed it already authorizes the new war against a new enemy. And the new draft gives no sense of our war aims, or what “victory” in a “war” against a non-state actor would look like.

Congress should seriously debate whether more, not fewer, limits are needed. No thumping of congressional chests; no questioning the patriotism of those who advocate restrictions. The three-year period is not, as some have suggested, a sop to the enemy; it’s a signal that after three years the Administration (whichever one it may be) had better be prepared to come to the Hill and talk once again about war aims. Debate in a democracy isn’t weakness; that’s as true once the guns are firing as it is before they start.

Once an authorization is passed, the “war power” has been invoked. Now is the time to debate it. The lack of the magic words will not excuse the U.S. later if rushes blindly into war now.

We want to hear what you think about this article. Submit a letter to the editor or write to letters@theatlantic.com.