Last month, the president stepped up American forces in Saudi Arabia to 500,000 armored, air supported, fully equipped combat personnel.

This month, the readiness of that force to engage Iraq in either limited or full-scale war is reported to be nearly complete. Should Jan. 15, 1991, pass without evidence of withdrawal of Iraqi divisions from Kuwait - divisions also reported at a strength of 500,000 armed personnel - 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 quite believe him to be serious. To make his position constitutionally correct, however, before presuming to move militarily against Iraq, he is obliged to secure the express endorsement of Congress - and he 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 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 commits 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 afterwards call on the country and Congress for support.

This very different arrangement was once commonplace in Europe under European monarchies. In England the king could - and did - command engagements in war. He did so according to his own notion of national or 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 of the American Constitution were intimately familiar with this history and they despised it. They strove to provide strongly against it in the making of the Constitution itself.

Precisely to avoid wars started by the executive that, once unchained, completely alter the option of policy available to Congress, the power to unchain the dog of war as an instrument of national policy was deliberately withheld from the president and transferred to the legislative branch instead.

As Thomas Jefferson declared in a letter to James Madison in 1789, commenting on the importance of the war declaration clause and its effect: "We have already given in example 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."

Without authorization by Congress, therefore, the president cannot constitutionally presume to use the forces now arrayed in Saudi Arabia to bombard Iraq. If he does, his orders could very appropriately be resisted, as indeed a strong argument can be made that they should.

Will the president seek such affirmative support from Congress for whatever measures he deems best to pursue, including war?

I devoutly hope he will. For otherwise he will provide citizens of constitutional faith with little hope in the integrity of their own government, and no reason to provide him any support.