PRESIDENT Bush has taken a logical step in the Gulf, but not the country's last. To leave forces in a defensive formation protected the Saudis but implicitly protected the Iraqis occupying Kuwait as well. Now by embracing the idea of an offensive option, harnessing it to the limited policy goals of the United Nations and ordering extra ground-force deployments, Mr. Bush gives the American-led Gulf coalition a more credible military threat to free Kuwait if Saddam Hussein otherwise refuses to withdraw. Preparation for offensive action will take several months; the Gulf clock is ticking.

How will President Hussein read this latest signal? He seems suspended between two conflicting propositions -- that the United States is inherently aggressive and that the United States lacks the heart to fight. If he settled on the former reading, he might yet take the Bush announcement as reason to attack before American and allied forces build up. If the latter, then the United States itself may come under tightening pressure to attack. Either way, the danger is being notched up.

Even as President Bush was speaking, the Kremlin was showing a new public tolerance for the use of force against Iraq as a last resort. That can only help Soviet diplomacy, which aims at achieving U.N. aims promptly and then working out the further containment of Saddam Hussein and Iraq. Earlier, President Bush had resisted submitting the issue of force directly to the Security Council, as Moscow has insisted, but now he wisely approves. It may not be necessary legally, but it's essential politically to strengthen the president's hand and to convey a message of seriousness to Iraq.

Something else that he must do now looms, something that would also make a powerful contribution. The approval that Congress gave in its fashion to President Bush's dispatch of units to the Gulf was an approval for defensive purposes only. Now, while Congress is out, he formally redefines their mission. This places on him an inescapable burden to ask Congress for its consent. The ways of informal consultation could be defended on grounds that the troops' mission was defensive and that hostilities were not necessarily imminent. But that is the case no more.

A congressional debate would have its risks, but the absence of the consensus it alone could provide would have far greater risks, namely of a disastrous gap between government action and public support. And congressional involvement alone could produce the kind of intensely sifted judgment suitable to a momentous national undertaking. A president and Congress respectful of the Constitution would not dodge such a debate. Given the months allotted to the buildup, there's time. A decision to face the contingency President Bush is now preparing for in which the two branches shared would be the best possible way to show Iraq that the United States means business.