Here are key events leading up to the referendum on whether Crimea should break away from Ukraine and join Russia.
Nov. 21, 2013: President Viktor Yanukovych’s government announces it is abandoning an agreement to strengthen ties with the European Union and instead seeking closer cooperation with Moscow. Protesters take to the streets.
Dec. 17: Russian President Vladimir Putin announces Moscow will buy $15 billion worth of Ukrainian government bonds and cut the price Ukraine pays for Russian natural gas.
Jan. 28, 2014: In concessions to the opposition, the prime minister resigns and parliament repeals harsh anti-protest laws that had set off violence.
Feb. 18: Protesters attack police lines and set fires outside the parliament after it stalls on a constitutional reform to limit presidential powers. Riot police respond to the violence by trying to push protesters off Independence Square. At least 26 people die and hundreds are injured.
Feb. 20: Hours after a truce is announced, violence between protesters and riot police resumes. More than 80 people, mainly protesters, are killed by gunfire.
Feb. 21: Under a European-mediated plan, protest leaders and Yanukovych agree to form a new government and hold an early election. Parliament slashes his powers and votes to free his rival, Yulia Tymoshenko, from prison. Yanukovych flees Kiev after protests flare.
Feb. 22: Parliament votes to remove Yanukovych and hold new elections.
Feb. 23: Pro-Russia protesters start rallying in Crimea, where Russia has a major naval base.
Feb. 26: Leaders of Ukraine’s protest movement propose legislator Arseniy Yatsenyuk as prime minister. In Moscow, Putin orders major military exercises just across the border.
Feb. 27: Masked gunmen seize regional parliament and government buildings in Crimea. Yanukovych is granted refuge in Russia.
Feb. 28: Ukraine says Russian troops have taken up positions around strategic locations on the Crimean Peninsula.
March 1: Troops under apparent Russian command take over Crimea without firing a shot. President Obama calls Putin to demand the troops’ withdrawal.
March 2: Ukraine appeals for international help, fearing a wider Russian invasion.
March 3: Ukraine says there are up to 16,000 Russian troops in Crimea. Russia says it has approved troop deployment at the request of Yanukovych.
March 6: Crimea’s parliament declares the region wants to join Russia and will let voters decide in a March 16 referendum.
March 14: A last-ditch diplomatic effort before the referendum fails in London, where Secretary of State John F. Kerry meets with Russian Foreign Minister Sergei Lavrov amid threats of sanctions against Russia if it annexes Crimea.
March 16: Referendum in Crimea.