WASHINGTON -- The United States enters uncharted territory as it formally begins a new era with a Western-looking but assertive Russia and the collection of smaller, potentially quarrelsome countries that made up the Soviet Union.
Recognizing Russia as the successor state to the Soviet Union, the United States thus ends its relationship with a onetime Communist giant that had been its World War II ally, Cold War adversary and, in its death throes, a cooperative partner.
The greatest dangers ahead lie less in Russia's dealings with the world at large than in its relations with the newly independent states surrounding it, according to U.S. officials and analysts.
"They've got a lot of problems at home. They're not going to fiddle around in the world," one U.S. official said.
The United States is expected tomorrow to recognize Russia as filling the Soviet international role. It also plans to open diplomatic relations with five other key republics but withhold ties from others until they show progress in democracy, human rights and border arrangements.
Russia's president, Boris N. Yeltsin, pleasantly surprised skeptics here when he quickly worked to smooth relations with Ukraine and form a Commonwealth of Independent States.
But if conflict arises on its borders, Russia may be tempted, as in the past, to settle matters by force. "Historically, when there has been instability on its periphery, Russia has expanded and taken control," one U.S. official said.
"Russian nationalism is a very serious disease," said Marshall Goldman, director of the Russian studies center at Harvard University and a professor at Wellesley College.
There will be a tendency among the republics, Mr. Goldman said, to try to draw the United States into taking sides in inter-republic disputes it previously didn't have to worry about, he said.
"We're going to find ourselves responding to one republic and getting criticism from others," he said.
Secretary of State James A. Baker III has publicly voiced fears of seeing the former Soviet Union become a warring Yugoslavia -- with nuclear weapons.
Leaders of Georgia and Azerbaijan, he noted Dec. 12, have succumbed to authoritarian tendencies. Mr. Goldman warns that other republics as well, as newcomers to a democracy in a vast former empire that has never enjoyed it, shouldn't automatically be counted in the democratic camp.
Senior U.S. officials have few illusions about the cohesive strength of the new commonwealth. They may or may not have common defense arrangements, one official said recently, but they won't have strong economic cooperation.
And the republics' respective leaders don't seem to appreciate fully the difficulties ahead in changing their systems, the official said.
Noting that most of the early reformers in Central and Eastern Europe are now out of power, this official said, "it is easier in Central and Eastern Europe than it is in the Soviet Union because you've got 40 years of history vs. 70 with a different system."
So far, with mixed success, the United States has held out the carrot of expanded Western aid and investment to republics that proceed democratically, adhere to human rights, respect borders and international obligations, and control and reduce their nuclear forces.
With Russia self-absorbed and desperate for Western assistance, several U.S. officials voiced confidence that even with nuclear weapons and a U.N. Security Council veto, it would continue the cooperation that marked Soviet dealings with the United States under Mikhail S. Gorbachev and his foreign minister, Eduard A. Shevardnadze.
"Economics will drive foreign policy," a senior U.S. administration official predicted.
Russia and the other republics are happy to be freeing themselves of the draining Third World entanglements that had poked American nerves, Mr. Goldman said. And with the emergence of independent states bordering the Middle East and Asia, there is less of a risk that Russian and U.S. interests will clash overseas.
As for Cuba, the senior U.S. official predicted, "They'll pull the rug out from under [Fidel] Castro."
But another official echoed fears within the State Department that sheer confusion and economic desperation may prolong some Russian ties to Third World dictatorships. Ukraine, he said, might be tempted into a barter arrangement of exchanging its foodstuffs and manufactured goods for tropical and citrus fruits from Cuba. Similarly, old relationships might bring a continued flow of technology to North Korea, which is suspected of trying to develop nuclear weapons, he said.
A major uncertainty is the unpredictable Mr. Yeltsin himself, a stark personality contrast with Mr. Gorbachev and Mr. Shevardnadze, who buried the superpower rivalry in cooperation and nurtured friendships with top U.S. leaders.
U.S. officials bristled on Mr. Baker's recent trip when the Russian leader upstaged and publicly embarrassed Mr. Gorbachev and Mr. Shevardnadze.
But the sheer speed and ruthless decisiveness Mr. Yeltsin has displayed may ultimately prove stabilizing. Said one senior administration official:
"The argument could well be made and is made that the certainty of knowing what is going to happen, knowing that there will be an end to the union, and that we are going to be dealing with 10 separate, independent nations, is important in terms of moving forward and in terms of stability."