Saturday, U.S. President George W. Bush will wrap up his European trip with what will be his most important negotiation: that with Russian President Vladimir Putin. It's Bush's first real game with the so-called big guns. It's also the first summit in a long time where the American president is going in as the underdog.

In his first five months in office, Bush appeared to be recovering nicely from the perceived political weakness stemming from his dead-heat election victory. But when Vermont Sen. James M. Jeffords left the Republican Party and the Republicans lost control of the Senate, this resurrected the realization of just how politically weak Bush actually is.

Putin, on the other hand, is getting steadily stronger in Russia. His physicians, very deliberately, released a report last week on his robust physical health and on the rigorous regimen he maintains, symbolically contrasting his condition to that of Bush. There is no doubt that for the first time since the fall of Soviet President Mikhail Gorbachev, Russia has a leader that is both in control of his own government and increasingly in control of the country. Putin's battles are far from over, but he clearly has the upper hand.

What does each side want? Bush wants Russia's support on National Missile Defense. He also wants to expand NATO eastward. While Russia no longer sees the West and a nuclear conflict as its primary national security threat, it is against abandoning the 1972 Anti-Ballistic Missile Treaty. And Putin is firmly against an eastward NATO expansion.

Bush cannot afford a failure or, to be more precise, he cannot afford anything appearing to be a failure. Bush has only recently beaten back his poor foreign-policy reputation. If he emerges Saturday with no score, it'll be like he's right back on national television being pounded by a local Boston reporter demanding that he name various world leaders. Conversely, there is no question in Russia or around the world of Putin's mastery of international relations.

That gives Putin a tremendous advantage. At the end of the summit, both sides will be working hard to define what happened. For the moment, Putin has more credibility than Bush, at least in the eyes of the global news media. That means that should Putin choose, he could subtly spin the summit as a dismal failure or even a disaster and easily shift the onus for the failure on Bush.

Bush's weakness is not all personal or political. The United States is for the moment in a tricky geopolitical situation. U.S. relations with China are tense, with subtle, if not very realistic, rumors of possible war circulating. It is extremely important for the United States that Russia not join China in an anti-American alliance, and even here U.S.-Russian relations have not gotten off to a good start.

One of the first decisions the new president made was to break up Russian espionage networks in the United States. From the extremely public arrest of the FBI agent Robert P. Hanssen to the mass expulsion of Russian diplomats after accusations of espionage, the Bush administration went directly into confrontation with the Russians. Immediately thereafter, the president signaled his commitment to seek revisions -- or possible unilateral abrogation -- of the 1972 Anti-Ballistic Missile Treaty to permit U.S. development of ballistic missile defenses. Those steps alone created substantial tensions in U.S.-Russian relations even before the crisis in American relations with China.

Bush therefore is facing an antagonized Russian leader at the time when he is seeking a very important and substantial offering from Putin: at minimum, Russian neutrality between the U.S. and China, and at most, a Russian commitment to side with the United States against China.

For his part, Putin does not seem to care that much about the issue of missile defense per se. He is aware, however, that the Bush administration has made this program -- which will ultimately have to win the support of Congress -- a key element of its defense policy. Putin is also very much aware that many Americans, not to mention U.S. allies, share Russia's opposition to missile defense. A Russian "nyet," might galvanize missile defense opponents in the United States, further weakening Bush.

Putin's highest priority is symbolized by the still-neutral ground on which he and Bush will meet. Slovenia very badly wants to be part of NATO, and the alliance actually needs Slovenia as a member. Putin wants the opposite -- a neutral zone between NATO and Russia (for instance where the airspace would not be used by NATO aircraft). From Putin's standpoint, extending NATO into the Balkans or into the Baltics is entirely unacceptable, as is American interference in the Caucasus, Ukraine or (if not sanctioned by Russia), in central Asia. Putin does not seriously expect financial aid from the West but he does expect at least tacit, if not rock-solid, respect for Russia's interests as a great power.

Putin must, therefore, make a critical and complex decision regarding his response to Bush. There is very little that Bush can do to hurt him right now. There is little aid flowing that Bush can use as a club. Even a Bush threat to proceed with further NATO expansion is something far from being solely in American hands. Even if the talks break down, Putin will appear to be a domestic Russian hero standing up to the American bully.

Bush can walk out of the Ljubljana summit with a missile-defense agreement and with Putin spinning him as the greatest statesman since Metternich. The price for this would be U.S. acquiescence (tacit if not spelled out in the communiqué) to an inviolable Russian sphere of influence that extends into southeastern Europe as far as the Balkans. Thus, what Putin wants from the Americans is a little Yalta Conference that will define the Russian sphere of influence and create a clear neutral zone between that sphere and NATO.

If the Americans balk on this, Putin can pull the rug out from under Bush. While the Russian president cannot break Bush politically, he can certainly make him appear ineffective and wreck his missile-defense plan. Far more important, Putin can opt to turn Russian policies eastward toward Beijing, causing the United States no end of grief.

For Bush, this is probably a pretty bad time for a summit meeting with the Russian president, although White House planners couldn't have known that the political ground would so unexpectedly slip out from under Bush's feet after the summit had been locked in. Still, no one is happier than Vladimir Putin, the first Russian leader since Leonid Brezhnev to hold the whip hand at a summit with the Americans.