Secretary of State James A. Baker III departs today on a mission to Europe and the Middle East that could prove far more challenging to his skills as a negotiator and persuader than anything he has yet attempted in domestic politics or international affairs.

His planned meeting Wednesday in Geneva with Iraqi Foreign Minister Tariq Aziz will be the first direct high-level contact between the United States and Iraq since the invasion of Kuwait five months ago. It also may be the last such contact before the Jan. 15 United Nations deadline for Iraq to vacate Kuwait.

The meeting is expected to be the centerpoint of the Baker mission during which the secretary of state will be trying to head off any last-minute flurry of third-party diplomacy that might encourage Iraq to keep its troops in Kuwait.

In many ways, the meeting with Aziz is different from other pressure-filled environments in which Baker has always thrived. He will not enjoy the kind of leeway that he has had in the past as a negotiator because this time he has been instructed not to negotiate. He also may find it necessary to throw open the curtains on his discussions rather than operate in the secrecy that he prefers as a tactician.

"This has got to be kind of a nightmare for him," said an associate who has worked closely with Baker in both diplomacy and domestic politics. "It is a no-win situation. He can't make any secret deals, but everyone will suspect that he has."

Moreover, Baker's possible avenues for progress in the talks were somewhat narrowed Friday when President Bush announced that Baker could not go on to Baghdad to see Iraqi President Saddam Hussein if the opportunity should arise. Bush said that option was no longer open because Saddam had refused to agree to the dates that Bush had proposed earlier for such a visit.

The president's statement in effect puts even more importance on the meeting with Aziz, who has been regarded by U.S. officials as more a messenger than a decision-maker in the Iraqi leadership.

In earning his reputation as a negotiator, Baker has always relied on a similar pattern of tactics. He believes in staking out a series of positions, sometimes at the outer boundary of what he really wants, then working, often in complete secrecy, to convince his adversary that Baker's positions are really in the adversary's best interests, too. Baker then makes concessions toward his opponent in an effort to close a deal, only going public with the terms when all the details have been agreed.

He did this, for example, when representing Ronald Reagan in the bargaining with Jimmy Carter's team over the climactic debate before the 1980 election. Baker opened with a preposterous position, that the debate be held on the eve of the election; he finally settled for a few days before the election, but close enough to suit the purposes of the Reagan campaign.

Even as secretary of state, Baker has tried to employ these tactics. He worked over many months, for example, to convince the Soviet leadership that a unified Germany within the North Atlantic alliance was really in the Soviet Union's interests. Over and over he looked for concessions that would allow the Soviets to accept this reasoning and make it easier for them to sell the concept to skeptical audiences at home. Ultimately, with generous offers of cooperation from West Germany, Soviet President Mikhail Gorbachev agreed.

But Baker goes into the talks with Aziz carrying a different kind of brief.

Bush and the other leaders of the coalition against Iraq already have ruled out any serious negotiations over the basic terms of the 12 United Nations resolutions calling for Iraq's complete and unconditional withdrawal from Kuwait. Baker has no license to make concessions to Aziz on the central demands, and this instruction effectively will be made clear in the letter from Bush to Saddam that Baker is expected to deliver.

Second, Baker is not sitting across the table from a real interlocutor who is in a position to make concessions. Aziz is not considered by U.S. intelligence agencies to be part of Saddam's inner circle but rather a reliable funnel back to Baghdad.

Thus, Baker has been limited in advance to the task of presenting, not negotiating, and doing it to an intermediary, not a principal.

Furthermore, officials said they do not think Iraq has reached the point where it is serious about negotiating an end to the crisis, but rather is looking for a way to forestall any attack by the international coalition arrayed against it. This calculation, if borne out, would seem to restrict further the scope of the discussion, suggesting that the talks would not address such delicate but important topics as how to begin a de-escalation of the massive armies facing each other on the Arabian Peninsula.

However, Baker's hands are not completely tied. Perhaps relying on his familiar approach of trying to articulate his opponents' "best interests," Baker is expected to tell Aziz that the coalition will not attack if Iraq pulls out. And if Iraq does not withdraw, Baker will stress, Saddam and Aziz will lose all in a war against a superpower and its partners. Baker may well emphasize anew that Bush is not bluffing about the use of military force.

Baker also is not proscribed from talking to Aziz about life after the crisis -- assuming Iraq quits Kuwait and providing Baker does not link the withdrawal to other issues. The United Nations resolutions contemplate Iraqi negotiations with Kuwait over disputed territory, and Baker has said the alliance anticipates the creation of new security structures in the region afterward. Although Baker cannot truly negotiate these matters in Geneva, a discussion of them may be significant in unlocking a diplomatic solution.

Finally, Baker often has used secrecy to avoid public pressure while he is negotiating, but in this case, officials said, he may find he is better served by staying in the open, thus insulating himself against any suspicion that he has made or suggested any private deals. "This is going to be very open diplomacy," said a senior official familiar with Baker's thinking.