LISBON, Portugal — In a year when Europe is marking a century since the outbreak of World War I, the images of helmeted, balaclava-clad young men fighting and dying among the burning barricades of Kyiv have a terrible resonance.
The bloodshed that erupted in Ukraine's capital and other cities, leaving at least 25 dead in the past two days, has triggered warnings of civil war in the country of 45 million. It leaves Europe facing its biggest threat of armed conflict since the Balkan wars of the 1990s.
Western leaders expressed mounting concern over the widening violence. The European Union is preparing to slap sanctions on the Ukrainian leadership after appeals for restraint failed to halt Tuesday's deadly crackdown on protesters.
More from GlobalPost: Live blog: Violence returns to Kyiv
Polish Prime Minister Donald Tusk cautioned that Ukraine could already be in the "first hours of a civil war," with dangerous spillover risks for its neighbors.
"Events in Ukraine will determine the history and future of the entire region," he told parliament in Warsaw, warning the unrest would have a "direct impact on the future and the security of Poland."
Tusk led calls for EU sanctions on the Ukrainian regime. Until now, other European leaders have been wary, concerned they could sink EU attempts to mediate between President Viktor Yanukovych and the opposition, or inflame Europe's relations with Yanukovych's backers in Moscow.
Tuesday's violence has pushed the EU into a tougher stance.
"We must be clear: ultimate responsibility for deaths and violence is with President Yanukovych," Swedish Foreign Minister Carl Bildt, tweeted. "He has blood on his hands."
French President Francois Hollande and other leaders quickly swung behind Tusk's call for sanctions. "Those who committed these acts and those who are planning more should know they will be punished," Hollande said in Paris on Wednesday.
An emergency meeting of EU foreign ministers in Brussels on Thursday is expected to impose travel bans and asset freezes on selected Ukrainian officials.
"These sanctions will show how seriously we are taking the defense of democratic rights," German Chancellor Angela Merkel told a joint news conference with Hollande.
Many fear, however, that the punitive measures and renewed European calls for compromise will come too late.
"As the regime now stands with its back to the wall, having already cut relations with the West, it is unlikely that sanctions will make a real difference in the calculations of Yanukovych and his inner circle," said Ulrich Speck, visiting scholar at the Carnegie Europe think tank in Brussels.
More from GlobalPost: Ukraine's revolution turns radical
This week's bloodletting is leading to a radicalization on both sides. Hardliners among the opposition issued a call for supporters to take up arms, and there's talk of the more pro-European west of the country breaking away. Yanukovych appears determined to crack down on opponents he accused of seeking to seize power by force, regardless of the consequences.
Officials close to Russian President Vladimir Putin have been encouraging Yanukovych to take a tougher line — and warning the Europeans not to interfere. The foreign ministry in Moscow was quick to blame this week's violence on Western meddling.
"The campaign of pressure on the Ukrainian authorities could well lead to a civil war starting there," said Alexei Pushkov, chairman of the foreign affairs committee in Russia's lower house of parliament. "A significant amount of responsibility for this falls on the West."
The wave of demonstrations began in November after Yanukovych pulled out of the scheduled signing of a sweeping economic agreement with the EU and instead accepted a $15-billion bailout from Russia.
Ukrainians took to the streets fearing their country was being dragged back into Moscow's orbit, 22 years after it broke away from the Soviet Union.
For the countries of the European Union, armed unrest in such a large and strategically important neighbor is a nightmarish prospect.
A conflict with widespread civilian casualties would lead to pressure for international action — as with the eventual NATO intervention in Bosnia and Kosovo.
In Ukraine, that could lead to direct conflict with Moscow, given that Putin has made clear he regards the country a matter of vital Russian interest. Few European nations would be willing to take that risk.
Economic conflict is another danger.
The EU is heavily dependent on Russian energy, with much of the oil and gas pumped westward through Ukrainian pipelines.
Almost a third of the EU's oil, gas and coal imports come from Russia. Some countries — like Finland, Slovakia and Bulgaria — are almost totally reliant on Russian gas. Germany, Britain and Italy all significantly increased Russian fuel imports last year.
If Moscow or Kyiv were to turn off the taps, European homes could be left without heating, and factories without power.
War in Ukraine could also trigger a massive refugee influx.
The 1990s fighting in former-Yugoslavia displaced an estimated 4 million people. Up to 800,000 of them fled to EU countries. Ukraine's population is more than 10 times bigger than Bosnia's and over 20 times that of Kosovo.
A surge of Ukrainian refugees could have a destabilizing effect, at a time when far-right politicians are making serious electoral gains in countries such as France, Austria and the Netherlands by campaigning against immigration from Eastern Europe.
Finding a way back from the brink will not be easy.
Many analysts say Western leaders should be turning their attention to Putin, warning him of the economic dangers of conflict — given that Russia needs Western markets as much as Europe needs its fuel supplies.
"Yanukovych now relies on the Kremlin. Therefore the West's messages must be directed to the Kremlin," Speck told GlobalPost. "Moscow must be warned by leaders in Berlin, Brussels and Washington that further escalation in Kyiv will lead to severe consequences in Russia's relations with the EU and the US."