Chronology of a crisis

November 2013: Former Ukrainian President Viktor Yanukovych unexpectedly refuses to sign an association agreement that had been negotiated since 2007 between Ukraine and the European Union, instead turning toward Russia. His decision triggers nationwide protests.

December 2013: Anti-government demonstrations are stepped up. Maidan Square in Kyiv becomes the focal point for protests against Yanukovych. Opponents of the government occupy the capital's city hall and union headquarters. Police injure numerous people during the demonstrations.

January 2014: The Ukrainian parliament approves several laws limiting the rights of protesters. Punishment for occupation and blockade of government buildings becomes harsher. Covering one's face during protests and publishing libelous content and "calls for extremism" on the Internet become crimes.

Protesters urge renewed action; hundreds of thousands take to the streets. Special security forces crack down on demonstrators in Kyiv. There are severe clashes between police and the opposition. After talks with the opposition, the government says toward the end of the month that it is prepared to dismantle some of the tough anti-protest laws. Parliament grants amnesty to imprisoned governmental opponents, while also demanding that protesters abandon occupied streets and government buildings.

February 2014: Security forces crack down ever harder on demonstrators. Former boxing world champion Vitali Klitschko, who takes part in street protests, becomes the opposition's most internationally recognized representative. The situation intensifies.

February 20, 2014: Violence escalates in Kyiv, and 80 are killed when the police open fire on demonstrators. The German, French and Polish foreign ministers launch an arbitration mission.

February 21, 2014: The government and opposition negotiate under EU and Russian mediation. Yanukovych announces new elections and offers to return to a 2004 version of the constitution that grants the executive fewer powers. Many in the anti-government movement do not see this as enough. Some call for resistance; others continue to demand Yanukovych's resignation. It becomes clear that the opposition is made up of several divergent groups, including staunchly nationalist elements. Opposition politicians including Vitali Klitschko sign a compromise with Yanukovych. But many continue protesting against the president at Maidan.

February 22, 2014: The Ukrainian parliament removes Yanukovych from power in a strong majority vote. He's accused of illegally appropriating mandates. New presidential elections are set for May 25. Yanukovych disappears. Parliamentarian Yulia Tymoshenko is released from prison after two-and- a-half years.

February 23, 2014: Parliament votes in Oleksandr Turchynov as the nation's interim president, who announces intentions for the country to follow a more Western course. Yanukovych, who has fled to Russia, is sought on charges of mass murder.

February 27, 2014: Parliament confirms pro-European politician Arseniy Yatsenuk as head of the transitional government. Russian President Vladimir Putin orders a major military exercise along the Ukrainian border. After fights between supporters and opponents of the new government in Kyiv, pro-Russian militias occupy the government's seat of power and parliament in the Crimean capital of Simferopol. Thousands of Crimean tartars demonstrate against splitting up the autonomous republic. Pro-Russian demonstrators demand closer ties to Moscow.

Early March 2014: Military personnel of unclear nationality take over strategic points in Crimea. Leaders in Kyiv claim that Russia stationed thousands of soldiers on the Crimean Peninsula. A referendum on the future status of the autonomous republic is set for March 30. Despite heavy protest from the West, Russia continues sending troops to Crimea. The EU and US announce plans for sanctions. The referendum is pushed forward again to March 16.

March 16, 2014: The controversial Crimea referendum results in a clear majority in favor of splitting from Ukraine. Putin signs a pact shortly thereafter to take the state into Russian possession.

March 24, 2014: Russia is kicked out of the G8, a global group of leading industrialized countries. A G8 conference planned in Sochi for June is cancelled. The United Nations criticizes the Russian annexation of Crimea. NATO suspends cooperation with Russia.

Early April 2014: Pro-Russian activists occupy regional administrative centers in the major cities of Kharkiv and Donetsk, demanding referenda for separation from Ukraine. A few days later, Interior Ministry troops clear an administrative building that had been occupied by activists. Although Russia denies it, NATO estimates as many as 40,000 Russian troops are stationed along the Ukrainian border. Moscow warns Kyiv to refrain from violence against pro-Russian activists, or there will be "civil war."

April 12, 2014: Armed pro-Russians in camouflage take over a police station and secret service office in the eastern Ukrainian city of Slovyansk; meanwhile, separatists occupy police headquarters in Kramatorsk and Donetsk.

April 13, 2014: Ukrainian soldiers begin an "anti-terrorist" campaign against separatists in Slovyansk. People are injured and killed. In the following days, pro-Russian forces seize tanks, and the campaign is broken off. Donetsk's city halls falls to separatists.

April 17, 2014: Ukraine and Russia reach an agreement in Geneva specifying that illegal groups in Ukraine are to be disarmed, occupied buildings are to be returned and amnesty granted. Parties do nothing to enact the agreement. German Foreign Minister Frank-Walter Steinmeier has since called for a second international conference in Geneva.

April 25, 2014: Eight military observers from the Organization for Security and Co-operation in Europe (OSCE), including four Germans, are taken hostage near Slovyansk. Two days later, they are presented to the media. Local militia leaders call them war prisoners and seek to trade them for comrades imprisoned in Kyiv. The EU and US enact new sanctions against Russia. Moscow threatens "painful consequences," while at the same time assuring it will not march into Ukraine.

May 1, 2014: Separatists storm the state prosecutor's building in Donetsk. There are violent clashes with police.

May 2, 2014: The bloodiest day since the violence in Kyiv: Nine die in a renewed military offensive against separatists in Slovyansk. Pro-Russian activists shoot down two Ukrainian military helicopters. In the southern Ukrainian port city of Odessa, more than 40 die in street clashes and in a fire in a contested union building.

May 3, 2014: After Russia intervenes, separatists release the OSCE observers. Military actions continue in Slovyansk and Kramatorsk.

May 4, 2014: Ukrainian government forces continue to fight separatists. New deaths and injuries are reported, and a number of cities are regained. Most pro-Russian activists have retreated to Slovyansk, among other locations. Ukrainian soldiers begin to encircle the city.