President Bush doesn't seem to care that Congress wants a bigger role in guiding the Iraq war. Talking about his plan to send in 20,000 additional troops, he said on "60 Minutes" that he knows Congress can vote against it, "but I've made my decision and we're going forward."

It is hardly the first time this president has insisted that he is "the decider," or even the first time he's used the Constitution to justify it, as Vice President Dick Cheney did when he told Fox News: "The Constitution is very clear that the president is, in fact, under Article 2, the commander in chief."

But Mr. Cheney told only half the story. Congress has war powers, too, and with 70 percent of Americans now opposed to President Bush's handling of the war, according to an ABC News/Washington Post poll, it is becoming more assertive about them. Congress is poised to pass a resolution denouncing the troop increase. Down the line, Congress may well consider mandatory caps on the number of troops in Iraq, or setting a date for withdrawal.

If it does, we may be headed toward a constitutional clash, with the administration trying to read powers into the Constitution — as it has with its "enemy combatant" doctrine and presidential "signing statements" — that the Founders did not put there. The Constitution's drafters were intent on balancing power so no one branch could drift toward despotism. The system of checks and balances that runs through the document divides the war power between the president and Congress.

The Constitution's provision that the president is the commander in chief clearly puts him at the top of the military chain of command. Congress would be overstepping if, for example, it passed a law requiring generals in the field to report directly to the speaker of the House.

But the Constitution also gives Congress an array of war powers, including the power to "declare war," "raise and support armies" and "make rules concerning captures on land and water." By "declare war," the Constitution's framers did not mean merely firing off a starting gun. In the 18th century, war declarations were often limited in scope — European powers might fight a naval battle in the Americas, for example, but not battle on their own continent. In giving Congress the power to declare war, the Constitution gives it authority to make decisions about a war's scope and duration.

The Founders, including James Madison, who is often called "the father of the Constitution," fully expected Congress to use these powers to rein in the commander in chief. "The constitution supposes, what the History of all Governments demonstrates, that the Executive is the branch of power most interested in war, and most prone to it," Madison cautioned. "It has accordingly with studied care, vested the question of war in the Legislature."

In the early days of the republic, the Supreme Court made clear that Congress could limit the president's war powers — notably in the Flying Fish case. In 1799, during the "Quasi War," the undeclared sea war between the United States and France, Congress authorized President John Adams to clamp down on trade between the two nations by stopping ships headed to French ports. But Adams went further, ordering commanders to stop ships that were sailing to or from a French port.

When the Flying Fish was seized while sailing from a French port — something Congress had not authorized — the ship's owner sued. The Supreme Court decided in his favor, ruling that the president had no right to issue the order he did. John Marshall, the nation's greatest chief justice, declared that even in a time of hostilities, a president's decision to act militarily beyond what Congress had authorized was "unlawful."

The court has repeatedly reinforced this principle. In 1952, in the steel seizure case, it ruled that President Harry Truman could not seize steel mills to avert a strike — even though steel was needed for the Korean War — because Congress had set out a different way of handling the labor unrest. More recently, in Hamdan v. Rumsfeld, it held that President Bush must follow Congressional guidelines when he sets up military tribunals for detainees.

Past Congresses have enacted just the sort of restrictions the Bush administration is trying to foreclose today. During the Vietnam War, the Foreign Assistance Act of 1974 capped the number of American military personnel in South Vietnam at 4,000 within six months. The Lebanon Emergency Assistance Act of 1983 required the president to get Congress's approval for any substantial increase in the number or role of armed forces in Lebanon.

There is little question that Congress could use its power of the purse to end a war. But cutting off financing is a drastic step, and one that members of Congress are understandably reluctant to take, because it can look like a refusal to support the troops. The Constitution's text, Supreme Court cases and history show, however, that Congress can instead pass laws that set the terms of military engagement. Whether it would be wise for Congress to adopt such limits is debatable; whether it has the authority to do so should not be.

The Bush administration insists that if Congress tries to manage the Iraq war, it will leave the commander in chief with too little authority. But the greater danger is the one Madison recognized at the nation's founding — that all the power will be left with the person "most interested in war, and most prone to it."

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