In Ukraine, the Kremlin poured more than $200 million into this fall's presidential election, in part to protect economic ties worth up to $10 billion a year. Putin made two trips to Ukraine before the election to boost the candidacy of Prime Minister Viktor Yanukovich, and huge billboards around Moscow urged Ukrainian expatriates to support Putin's choice. His opponent, Viktor Yushchenko, is a strong advocate of Ukrainian membership in NATO and the European Union.
Ukraine, with a population of 48 million, is the second-largest country in Europe and transports 90% of Russian gas to Europe. Many of the decisive battles of European history have been fought on its fields, and a westward-tilting Ukraine could sever Russia's access to its Black Sea Fleet, which is currently under a lease agreement with Kiev.
Moreover, Ukraine is the key to Russia's hope of establishing an economic coalition of former Soviet nations as a front against Europe. Belarus and Kazakhstan are also part of a pact initialed this year.
For both East and West, Ukraine always has been a "pivotal state" in what former U.S. national security advisor Zbigniew Brzezinski describes as the "Grand Chessboard" of geopolitics. That is especially true for Moscow, he said.
"Its very existence as an independent country helps transform Russia," Brzezinski said. "Without Ukraine, Russia ceases to be a Eurasian empire."
The dispute in Ukraine also could have implications for Putin's grip on power. The techniques that have enabled the Kremlin and its allies to determine election outcomes in Chechnya and Belarus are on the brink of failure in Ukraine. Most analysts say that would inevitably encourage Russia's own democracy advocates and threaten Moscow's elite.
The U.S. and its allies are reluctant to talk about Russia as a military adversary. "Nothing we know about Russia, nothing we see and feel in the cooperation that's going on, would suggest we should see Russia as anything but a partner of NATO," an alliance spokesman said.
But military planners, perhaps on both sides, remain prepared for the possibility that one day, a leader more aggressive than Putin will take the reins of a reinvigorated Russia. This is not unimaginable in a country in which polls show that the 1970s, when Leonid I. Brezhnev led the Soviet Union, are regarded as a golden era.
In an address to the Senate Intelligence Committee in February, former CIA Director George J. Tenet listed Russia, along with North Korea and China, as a "pivotal state."
"The Kremlin's increasing assertiveness is partly grounded in a growing confidence in its military capabilities," Tenet said, noting that although the Russian military remained at "a fraction" of its former strength, training rates and defense spending were increasing.
Russia's updated military doctrine makes it clear that the Kremlin, like Washington, is prepared to use preemptive strikes against other nations to protect itself, and also to resort to nuclear force if gravely threatened.
The installation of NATO military infrastructure in the Baltics would prompt Russia to "conduct its policy and military planning based on the principles of self-defense," Defense Minister Ivanov warned in a visit to Washington in April.
Russia's newly beefed-up nuclear weapons also provide the Kremlin with an important political tool.
"Clearly, nuclear weapons are a shield against potential U.S. sanctions, military or otherwise," said Trenin, the Carnegie official who has written a book on the Russian military. "The U.S. will not attack a nuclear power."
Both sides have thought through the logistics, if only theoretically.
In a report prepared by the Rand Corp. for the U.S. Army this year, one scenario explored the possibility of a conflict between Russia and NATO in the Baltics. The report referred to "an assumed decline in Russian-NATO relations in the period after 2007," and weighed how hard it would be for NATO to respond if Russian troops speedily overran the Baltics and dug in to wait for negotiations.
The report, analysts say, measures military capabilities; the chance of Russia invading the Baltics, most agree, is nearly zero.
Russia, for its part, conducted a military exercise in 1999 that envisioned "enemy" forces taking over Kaliningrad, the wedge of Russian territory between Poland and Lithuania that is the headquarters of the Baltic Fleet, and striking nuclear power plants and other targets inside Russia. As part of the exercise, a pair of Tupolev bombers simulated nuclear cruise missile strikes on the U.S. East Coast and Europe.
Nikolai Sokov, senior research associate at the Monterey Institute of International Studies, wrote in a report this year that Russia appeared to be using the possibility of limited nuclear strikes as a deterrent against political and military pressure.
But Russian officials say it is a mistake to confuse Moscow's assertion of legitimate interests with a return of empire-building.
"It is wrong to interpret any of this as Russia's attempts to impose its influence," said Igor S. Ivanov, the National Security Council secretary. "In these countries, all the generations of people, although they are people of different nationalities, lived in one state. They had common culture, common education, they worked together, they developed their economies together. If you please, common thinking was formed. Naturally, these are not some artificial ties, these are real ties that connect us."
At the same time, Russia's reemergence as a player is not to be discounted, they warn.
"Whereas American interests extend thousands of miles, and to many continents, let's accept that Russia has natural interests in the former Soviet states. Let's have a dialogue about this," former Soviet President Mikhail S. Gorbachev said in an interview.
"I think we have a unique chance to create a new quality of relations with the West. But we don't want to be beggars. We don't want to be treated by the EU or by the United States like we are down; that is something we will not accept," he said. "Russia will not be scared. Russia will not be intimidated."