The deepening crisis in Ukraine is likely to have a lasting impact on international relations, with a newly-assertive Russia forcing the United States and the EU to take a hard look at where they stand, analysts say.
Russia under President Vladimir Putin has played its hand well in Ukraine, smoothly taking Crimea from what was a former Soviet republic despite dire warnings of sanctions and worse from Washington and Brussels.
US President Barack Obama has taken a harder line than the 28 EU member states, hit by divisions over how far to risk ties with one of the bloc's largest trade partners and supplier of more than a quarter of its gas needs.
Thomas Gomart at the French Institute of International Relations (IFRI) says Putin has left Western powers "flabbergasted".
United perhaps in public, European Union leaders behind closed doors have quite different positions on how best to meet the challenge.
When signing the political provisions of a landmark EU association accord Friday, Ukraine interim premier Arseniy Yatsenyuk repeatedly warned Europe to speak with "one, single voice" in opposing Moscow.
The EU's biggest headache is to find a way of sanctioning Russia effectively, without causing any problems for the bloc's own, still convalescing economy.
"That will be difficult, it will take time and it will cost money," said Xavier Follebouckt, an international relations expert at the Catholic University of Leuven in Belgium.
US takes lead
Washington meanwhile has been forced back into taking the lead in Europe, with Obama's much-touted pivot to Asia knocked off course again. The US has had to reassure former Soviet states that their Western allies will stand by them.
US F-16 fighter planes have been deployed to Poland while Vice President Joe Biden recently visited Warsaw and the Baltic states in an effort to calm nerves.
Many Eastern European states have joined the EU and the US-led NATO military alliance since 2004.
Ukraine is a partner of NATO and was offered membership in 2008 but now-ousted pro-Russian president Viktor Yanukovych ditched that option in 2010.
"The United States finds itself on the front line," said Bertrand Badie, an international relations specialist at Sciences Po, one of France's leading political science research institutions.
Badie said it was significant that Obama chose to announce the latest US sanctions against Russia just as EU leaders were meeting in Brussels Thursday to discuss their next move.
That happened too with the first series of sanctions earlier this month.
Moscow appeared unimpressed, however, and announced its own tit-for-tat sanctions, ignoring warnings that it must de-escalate and seek a diplomatic solution.
"For Moscow, this is clearly brinkmanship" with the US, IFRI's Gomart says.
Putin is now "testing how far he can go", says Xavier Follebouckt, from the Catholic University of Leuven, noting that Western leaders have repeatedly ruled out a military response to Russia's annexation of Crimea.
The link between Russia's military spending and its gas exports was not lost on Ukraine's Yatsenyuk on Friday as he urged EU leaders not to rule out tougher economic sanctions against Moscow.
"They are selling gas and oil, mainly to the European Union; they take euros, dollars and pounds and buy armaments and vehicles, tanks, and invade an independent country -- this is the wrong business," Yatsenyuk said.
No end in sight
The Catholic University of Leuven's Xavier Follebouckt says the "crisis is not over", noting that Putin would not give up his dream of building a "Eurasian union".
Putin, after all, has called the collapse of the Soviet Union the "greatest disaster of the last century".
But while Russia has already signed deals with Belarus, Kazakstan and Armenia, without Ukraine on board, "the plan will collapse and Russia will feel weakened", Follebouckt says.
At the same time, the Russian economy is hardly growing and remains heavily dependent on oil and gas exports.
Western sanctions or efforts to cut energy dependence on Russia -- also given a boost at the EU summit -- may cut both ways but Moscow cannot be entirely indifferent.
There is also a risk that Russia could be outflanked on the trade front.
It is not party to the two largest trade deals currently being negotiated -- the Transatlantic Trade and Investment Partnership (TTIP) between the EU and the US, and the Trans-Pacific Partnership (TPP), which would link North America with the fast-growing Asian economies.