A WEEK from today, when President Clinton and Russian President Boris Yeltsin toast the 50th anniversary of their countries' triumph over the Nazis, their speeches and embraces will doubtless be warm and heartfelt.
The morning after, there will be a rude awakening.
After the May 9 ceremonies, Mr. Clinton and Mr. Yeltsin will sit down May 10 for difficult talks, and hopes are not high in Moscow that the summit will clear the air. Even if it is relatively successful, Russian and foreign analysts say, the euphoria that infused Moscow's attitude toward the United States in the wake of the Soviet Union's collapse can never be recaptured.
''Russian-American relations have gone from good to bad to worse, and the prospects are that they will get worse still,'' says Viktor Kremenyuk, deputy director of the influential USA-Canada Institute here. Moscow is worried by NATO's plans to expand in Eastern Europe, while Washington objects to Russia's deal to sell nuclear reactors to Iran and criticizes the war in Chechnya.
A certain souring of relations was only to be expected, argues one Western diplomatic observer. ''Objectively, Russia and America have different interests,'' he points out. ''Sometimes they will overlap, sometimes they won't, and sometimes they will conflict.''
Those differences have been accentuated, however, by the nationalist winds that have carried Russian foreign and domestic policy away from the pro-Western tack it held after the collapse of the Soviet Union in 1991.
A defiant mood, both among the electorate and leading political circles, means that ''Boris Yeltsin has lost a lot of his maneuverability, because the political center of gravity'' has shifted, Mr. Kremenyuk suggests.
At the same time, Russian officials know that Clinton himself can hardly afford any compromises: He is under fire at home for coming to Moscow even though Russian troops have killed thousands of civilians in their campaign against separatist rebels in Chechnya.
The US president is expected to counter his Republican critics by openly expressing Washington's displeasure at Russia's brutality in crushing independence-minded Chechens.
Yeltsin, however, has shown no signs that he is ready to alter his policy in the mainly Muslim republic beyond ordering a 10-day moratorium on fighting to try to lessen the embarrassment for his foreign Victory Day guests.
On the defensive over Chechnya, the Russian leader is likely to press his case that Moscow views any expansion of NATO into former Warsaw Pact territory as potentially threatening.
Russia has refused to join NATO's European security arrangement, the Partnership for Peace program, until Russia and current NATO members agree how far east NATO will expand. Western leaders, meanwhile, have insisted Moscow cannot dictate who can join their alliance.
Yeltsin hinted after a telephone conversation last week with Clinton that ''we are nearing a solution that would be acceptable to both parties,'' but said that any deal would have to be hammered out at the summit itself.
Even so, Russia appears to have overridden one key provision of the Conventional Forces in Europe treaty that limits deployment of troops and weaponry in Central and Eastern Europe.
Russian military leaders have already said Moscow will not abide by this treaty, arguing that recent unrest in the Caucasus has made the agreement an anachronism.
''The interests of Russia's security and territorial integrity should prevail over fulfilling the [treaty] document to the letter,'' Russian ground forces commander Col.-Gen. Vladimir Semyonov said.
Such nationalist concerns also appear to be driving Moscow's policy on its planned $800 million sale of up to four nuclear reactors to Iran.
Arguing that Iran is an outlaw terrorist state, the US administration has been putting intense pressure on Moscow to drop the sale, on the grounds that Tehran could use the Russian technology to advance its alleged nuclear-weapons program. On Sunday, Clinton announced the US would cut off all trade and investment to Iran, partly to demonstrate to Russia the urgency that the US feels about Iran's nuclear threat.
But the Russian government has refused to budge, pointing out that Washington is supporting the sale of identical technology to North Korea and arguing that Russia's nuclear industry needs the money that the sale would bring.
IN 1993, when Washington objected to a planned sale of Russian rocket engines to India, Moscow backed down. ''Two years ago, when the two things were put on the scales, American connections immediately outweighed Russian interests in India,'' recalls Kremenyuk. ''This time it is vice versa.''
Moscow is expected to meet one American demand, however -- to formally halt weapons sales to Iran. Russia promised some time ago to sell no more arms to Tehran once it fulfills a current contract for Kilo-class submarines.
There are limits, nonetheless, to Russia's more assertive foreign policy, whether it be in Serbia and Iraq -- where Moscow is trying, against Washington's will, to lift United Nations embargoes -- or in the former Soviet republics.
But Russia cannot afford to lose US support in forums where such backing really counts -- such as the International Monetary Fund, whose loans are critical to Russia's financial stability.
''The government needs to carve out an independent foreign policy, but without risking a rupture with Washington,'' says the Western diplomat. ''Following Washington's lead is not acceptable to people here, but in the end they can't afford to lose America's support.''