THIS WAS NO sudden, scared-up, uninformed Tonkin Gulf vote that Congress took on Iraq yesterday. Nor was it a hasty decision made simply on three intense days of floor debate. For no less than five months Congress has been profoundly immersed in this issue. In the end, it made a choice the weightier for all the time, information, scrutiny and debate that had gone into it. The choice was to add the warning of war to the embargo, diplomacy and buildup already being applied in order to strengthen the president's strategy of applying maximum pressure in the countdown to Jan. 15. In arming the president with legislative authority for his policy, Congress took the grave, responsible and necessary chance that this will help incline Saddam Hussein to quit Kuwait without war. President Bush struck the right note in his press conference after the voting in expressing the hope that this powerful new affirmation of American will would finally break through to the Iraqi leader.

As Jan. 15 nears, other countries and actors, including many who doubted the efficacy of American pressure, are using it to make a last-minute diplomatic surge. U.N. Secretary General Javier Perez de Cuellar has now brought to Baghdad the specific diplomatic charge of the European Community and the fervent hopes of many other nations and individuals. In the American threat to lead a coalition into battle, he has one tool of persuasion. He also carries positive incentives: American-supported offers to spare Iraq attack and to substitute peacekeeping forces for the foreign contingents now in Saudi Arabia, and a European pledge to call a regional peace conference.

It bears underlining, however, that the first ingredient for diplomatic progress -- prompt unqualified Iraqi withdrawal, or as Mr. Bush said yesterday, instant large-scale commencement of it -- is still missing. Saddam Hussein's aides churn out rumors of future compliance, but only the man himself counts, and he insists he will never yield Kuwait.

Between the European and American positions lie several variations. The Europeans, for instance, are readier to blur the no-linkage line: they would promise a conference on Israel and Palestine even before Iraq has agreed to withdrawal, let alone started or completed it. Whether the diplomats want Baghdad to notice these variations is uncertain. What must be made clear, however, is that no incentives can be made available to Iraq simply on the word of President Hussein. His concrete delivery on withdrawal must be the test.

The United States is long on record as supporting a "properly structured" international conference "at an appropriate time" -- hedges inserted to satisfy Washington's essential concern for Israel and for its own credibility, if a conference would address the American interest in no-linkage Mideast peace. But the United States is also on record as rejecting any arrangement smacking of compensating Iraq for aggression. This defines the narrow space in which Mr. Perez de Cuellar must work in Baghdad.