Russian forces have fired missiles at several Ukrainian cities and landed troops on its south coast, after Russian President Vladimir Putin authorised what he called a special military operation in eastern Ukraine.
Here is a timeline of the main events in Ukraine's political history since it won independence from Moscow in 1991.
1991: Leonid Kravchuk, leader of the Soviet republic of Ukraine, declares independence from Moscow. In a referendum and presidential election, Ukrainians approve independence and elect Mr Kravchuk president.
1994: Leonid Kuchma beats Mr Kravchuk in a presidential election deemed largely free and fair by observers, and he is re-elected in 1999 in a vote considered to be riddled with irregularities.
2004: Pro-Russian candidate Viktor Yanukovich is declared president but allegations of vote-rigging trigger protests in what becomes known as the Orange Revolution, forcing a re-run of the vote. A pro-Western former prime minister, Viktor Yushchenko, is elected president.
2005: Mr Yushchenko takes power with promises to lead Ukraine out of the Kremlin's orbit, towards NATO and the EU. He appoints former energy company boss Yulia Tymoshenko as prime minister but after in-fighting in the pro-Western camp, she is sacked.
2008: NATO promises Ukraine it will one day join the alliance.
2010: Mr Yanukovich defeats Ms Tymoshenko in a presidential election. Russia and Ukraine clinch a gas pricing deal in exchange for extending the lease for the Russian navy in a Ukrainian Black Sea port.
2013: Mr Yanukovich's government suspends trade and association talks with the EU in November and opts to revive economic ties with Moscow, triggering months of mass rallies in Kyiv.
2014: The protests, largely focused around Kyiv's Maidan square, turn violent. Dozens of protesters are killed.
February 2014: The parliament votes to remove Mr Yanukovich, who flees. Within days, armed men seize parliament in the Ukrainian region of Crimea and raise the Russian flag. Moscow annexes the territory after a 16 March referendum which shows overwhelming support in Crimea for joining the Russian Federation.
Maidan activists clashed with Ukrainian National guard soldiers in front of a burning barricade during protest against Ukrainian authorities. Source: EPA / SERGEY DOLZHENKO/EPA
April 2014: Pro-Russian separatists in the eastern region of Donbas declare independence. Fighting breaks out, which has continued sporadically into 2022, despite frequent ceasefires.
May 2014: Businessman Petro Poroshenko wins a presidential election with a pro-Western agenda.
July 2014: A missile brings down passenger plane MH17 en route from Amsterdam to Kuala Lumpur, killing all 298 people on board. Investigators trace back the weapon used to Russia, which denies involvement.
Russia has denied involvement in the shooting down of Malaysia Airlines Flight MH17. Source: AAP
2017: An association agreement between Ukraine and the EU opens markets for free trade of goods and services, and visa-free travel to the EU for Ukrainians.
2019: Former comic actor Volodymyr Zelenskyy defeats Mr Poroshenko in an April presidential election on promises to tackle corruption and end the war in eastern Ukraine. His Servant of the People party wins a July parliamentary election.
US President Donald Trump asks Mr Zelenskyy in July to investigate Joe Biden, his rival in the US presidential race, and Mr Biden's son Hunter over possible business dealings in Ukraine. The call leads to a failed attempt to impeach Mr Trump.
January 2021: Mr Zelenskyy appeals to Mr Biden, now US president, to let Ukraine join NATO.
February 2021: Mr Zelenskyy's government imposes sanctions on Viktor Medvedchuk, an opposition leader and the Kremlin's most prominent ally in Ukraine.
Spring 2021: Russia masses troops near Ukraine's borders in what it says are training exercises.
October 2021: Ukraine uses a Turkish Bayraktar TB2 drone for the first time in eastern Ukraine, angering Russia.
Autumn 2021: Russia again begins massing troops near Ukraine.
Pro-Russian protesters take photos of Ukrainian soldiers sitting on their armored vehicle in Kramatorsk
7 December 2021: Mr Biden warns Russia of sweeping Western economic sanctions if it invades Ukraine.
17 December 2021: Russia presents detailed security demands including a legally binding guarantee that NATO will give up any military activity in eastern Europe and Ukraine.
14 January 2022: A cyberattack warning Ukrainians to "be afraid and expect the worst" hits Ukrainian government websites.
17 January 2022: Russian forces start arriving in Belarus, to the north of Ukraine, for joint drills.
24 January 2022: NATO puts forces on standby and reinforces eastern Europe with more ships and fighter jets.
26 January 2022: Washington presents a written response to Russia's security demands, repeating a commitment to NATO's "open-door" policy while offering "pragmatic" discussions of Moscow's concerns. Two days later, President Vladimir Putin says Russia's main security demands have not been addressed.
Russian President Vladimir Putin. Source: Getty / TASS
2 February 2022: The United States says it will send 3,000 extra troops to Poland and Romania to help shield NATO allies in eastern Europe from any spillover from the crisis.
4 February 2022: Mr Putin, at the Beijing Winter Olympics, wins Chinese support for his demand that Ukraine not be allowed to join NATO.
7 February 2022: French President Emmanuel Macron sees some hope for a diplomatic resolution of the crisis after meeting Mr Putin in the Kremlin. Mr Macron then visits Kyiv and praises the "sang-froid" of Mr Zelenskyy and the Ukrainian people.
9 February 2022: Mr Biden says "things could go crazy quickly" as the US State Department advises Americans in Ukraine to leave immediately. Other countries also urge their nationals to leave.
A Russian sniper soldier during a tactical drill in Tambov Oblast, Russia. Source: Getty / Getty Images/Russian Defence Ministry Handout
15 February 2022: Russia says some of its troops are returning to base after exercises near Ukraine and mocks Western warnings about a looming invasion. Russia's parliament asks Mr Putin to recognise as independent two Russian-backed breakaway regions in eastern Ukraine.
21 February 2022: Mr Macron says Mr Biden and Mr Putin have agreed in principle to a summit over Ukraine.
In a televised address, Mr Putin says Ukraine is an integral part of Russian history, has never had a history of genuine statehood, is managed by foreign powers and has a puppet regime. Mr Putin signs agreements to recognise breakaway regions in eastern Ukraine as independent and order Russian troops there.
22 February 2022: US, UK and their allies enact sanctions on Russian parliament members, banks and other assets. Germany halts final certification of the Nord Stream 2 pipeline that was still waiting for approval.
Mr Putin, in a television address, demands Ukraine demilitarise and says the Minsk peace agreement over breakaway republics no longer exists, blaming Kyiv for killing the deal.
23 February 2022: Russian-backed separatist leaders ask Russia for help in repelling aggression from the Ukrainian army.
24 February 2022: Russian President Putin authorises "special military operations" in eastern Ukraine and asks Ukrainian forces to lay down their arms in a televised address. Russian forces begin missile and artillery attacks on Ukrainian forces and air bases, striking areas in major cities.
