Nothing in the Constitution allows a president to start a war and then call on the country and Congress for support.

Last month the president stepped up American forces in Saudi Arabia to 350,000 armored, air supported, fully equipped combat personnel. This month the readiness of that force to engage Iraq in either limited or in full-scale war is reported to be nearing completion. Should Jan. 15, 1991, pass without evidence of withdrawal of Iraqi divisions from Kuwait, the United States may find itself at war.

The undertaking of that war, we are told, may be at our initiative and not the initiative of Saddam Hussein. We have moved from a position of defense of Saudi Arabia to a readiness to "implement" the resolutions of the United Nations by force of arms. The president has so declared, and I believe him to be serious. To make his position constitutionally correct, however, he must secure the express endorsement of Congress before presuming to move militarily against Iraq, and he must must secure it in advance.

The power of the president as commander in chief of our armed forces is not to put the country into a war as he chooses. Rather, it is the power to pursue such war as Congress shall expressly authorize pursuant to the power reserved solely to Congress in accordance with Article I of the Constitution. The war-authorizing powers invested in Congress commit to that body, and not to the president, these fundamental political responsibilities and powers: to declare the reasons for war and the authority of the president as commander in chief in the conduct of that war. Nothing in the Constitution empowers the president to start up a war -- and afterward call on the country and Congress for support.

This very different arrangement was once commonplace under European monarchies. It was an arrangement the Framers carefully considered and expressly rejected in the Constitution of the United States. In England, the king could -- and did -- command engagements in war. He did so very much according to his own notion of national or of personal policy. And having committed the realm to war, he was then more able to force Parliament to provide support for that war, failing which it would have to face responsibility for such losses as then might occur in the field.

The Framers were intimately familiar with this history, and they despised it. Precisely to avoid executively initiated wars that, once underway, completely alter the options available to Congress, the power to let slip the dogs of war as an instrument of national and international U.S. policy was deliberately withheld from the president as commander in chief and transferred to the legislative branch. The Constitution requires that in moving to war (as distinct from repelling sudden attacks or resisting invasion), the initiative must first be expressly approved by Congress. As Jefferson declared in his letter to Madison in 1789, commenting on the importance of the war declaration clause and its effect: "We have already given in example {i.e., in this clause} one effectual check to the dog of war, by transferring the power of letting him loose from the Executive to the Legislative body, from those who are to spend to those who are to pay."

And if Congress fails to authorize, the president may not claim authority to engage our armed forces in war from the United Nations, as some have alleged. The United Nations has no standing under our Constitution to confer on the president a power he otherwise lacks -- i.e., it has no power to substitute itself for Congress in determining whether this country is to go to war. So the whole argument is absurd.

Moreover, the United Nations has not claimed otherwise. The U.N. resolution of Nov. 29 does no more than authorize "member states" to "use all necessary means" those states deem appropriate to help secure compliance with previous U.N. resolutions if Iraq does not comply with the previous resolutions by Jan 15. Whether those means are to be war rather than continuation of the collective economic blockade, or any other form of sanction, or nothing at all, is left to each state to decide. Engaging American armed forces in war, therefore, is an issue for the constitutional processes of the United States and does not come from the United Nations at all. The question is obviously and squarely one for us to resolve. It involves our armed forces, not those of the United Nations.

This is clearly not a circumstance when the president must act at once to repel an invasion or a sudden attack. Hundreds of thousands of armed Americans are in place in Saudi Arabia getting ready to launch a massive offensive. In this setting, the president must secure authorization from Congress for the enforcement of our demands by war. If he fails to do so and nonetheless embarks on war, it will not mark some trivial trespass on the margins of constitutional nicety. It will mark the boldest usurpation of power by a president we will have seen in this country since Watergate, and it should be treated in the same way. The stakes here are not simply economic, or military or geopolitical. The stakes are those of basic constitutional integrity and the rule of law. The writer is Perkins professor of law at Duke University.