A view of Ukraine's national flag waves above the capital with the Motherland Monument on the right, in Kyiv. Photo / AP
Today's invasion follows deterioration of relationship in the 30 years since Kyiv declared independence.
This is one of the world's most tense borders — where little more than 30 years ago no frontier existed. Ukraine and Russia were part of a vast Soviet Union, whose dissolution in 1991 has shaped the two countries' relations ever since.
For many observers in Ukraine and the west, it is a history defined by the Kremlin's desire to retain influence in what it calls its "near abroad". For Russia's President Vladimir Putin, it has been coloured by Nato's expansion east, which he claims is a security threat to Russia.
Below are some key events that have led to the rupture between the two countries.
Ukraine's experience of the Soviet Union (USSR) was marked by the holodomor, when Soviet leader Joseph Stalin's policies led to a mass famine in the early 1930s that Ukraine historian Serhii Plokhy said killed about 4 million in two years.
Kyiv declared independence from the Soviet Union in August 1991. On December 1 that year, Ukrainians participated in a referendum on the issue, with more than 90 per cent voting for independence.
Home to much of the USSR's agricultural production, defence industries and military, the withdrawal of the second-most populous republic was decisive for the bloc. "Ukraine freed the rest of the Soviet republics still dependent on Moscow," Plokhy wrote in The Gates of Europe. It "spelt the end of the Soviet Union."
In the first days of December 1991, Russia was still part of the USSR, but its president, Boris Yeltsin, was convinced its future lay outside the bloc, which was still led by his rival, Soviet leader Mikhail Gorbachev. A day after Ukraine's referendum, Yeltsin's Russia recognised Ukraine as an independent state.
Crucially, Moscow recognised Ukraine's borders as including Crimea, the multi-ethnic, multilingual Black Sea peninsula, which was transferred from Russia to Ukraine as a "gift" by Soviet leader Nikita Khrushchev in 1954 in an effort to win Kyiv's loyalty.
"For Yeltsin it was truly a politically remarkable step," said Vladislav Zubok, professor of international history at the London School of Economics.
Yeltsin believed moving fast would hasten both the demise of the bloc, and Gorbachev's resignation. It did both within weeks.
However, the question of Crimea, which held symbolic significance for Russians and Ukrainians, could not be put on hold, as Yeltsin had hoped. A dispute broke out almost immediately over the future of the Soviet Black Sea Fleet.
When the Soviet Union fell apart, many of its nuclear weapons ended up scattered across newly independent states. Kyiv held the majority of the Soviet arsenal outside of Russia, including 1,900 strategic nuclear weapons designed to strike the US.
"When Ukraine became independent . . . it was not only born nuclear, it was born the third-biggest nuclear power in the world," said Mary Elise Sarotte of Johns Hopkins University, author of Not One Inch, a history of Nato expansion in Europe.
A diplomatic push led to the 1994 Budapest Memorandum, in which Ukraine gave up its nuclear weapons and sent the warheads to Russia for destruction.
In return, the US, Britain, and Russia pledged to "respect the independence and sovereignty and the existing borders of Ukraine" and "refrain from the threat or use of force against [its] territorial integrity or political independence". Moscow violated this pledge when it annexed Crimea.
In autumn 2004 hundreds of thousands of Ukrainians took to the streets to protest against an election result they believed had been rigged to favour a candidate backed by Moscow.
Pro-western Viktor Yushchenko, whose campaign colours were orange, had emerged as the frontrunner with a platform to crackdown on the oligarchic economic system that had taken hold in the preceding decade, when state assets were privatised.
But despite exit polls showing his victory, the election commission gave the presidency to Viktor Yanukovych, former governor of the Donetsk region and Putin's preferred candidate.
A winter of protests on Kyiv's central square led to a vote rerun and the victory of Yushchenko, who had been poisoned and permanently disfigured during the campaign.
The Kremlin said the "Orange Revolution" was part of a plot by foreign intelligence services — and a dress rehearsal for regime change in Russia itself. Ukrainians began looking west.
In 2008, as Ukraine's new leaders worked on turning the country to face the west, Yushchenko sought a Nato Membership Action Plan (MAP) for Ukraine, a major step towards joining the alliance. The US supported the idea.
But by then, after Poland joined Nato in 1999, and the Baltic states, Bulgaria and Romania in 2004, Putin viewed the transatlantic alliance's expansion as a smokescreen for attempts to contain Russia. He threatened to point nuclear weapons at Ukraine should it proceed, describing the young democracy as "not even a country" and a part of Russia.
As a result several Nato members stymied Washington's push to grant Ukraine and Georgia Nato MAPs. Instead, a promise was penned at a summit in Bucharest in April. "We agreed today that these countries will become members of Nato," the alliance said in a declaration.
By August, Russia was at war with Georgia. In the four-day invasion, Moscow seized two breakaway territories, to foster the sort of conflict that would make it difficult for the country to join Nato.
In 2010 in Kyiv, Yanukovych — a former prime minister — became president. He quickly dropped Ukraine's Nato bid, in a big concession to Moscow.
Despite their government's close ties with Moscow, many Ukrainians continued to see their future within Europe.
When, in 2013, the opportunity came to join the EU's Eastern Partnership programme — set up to bring Ukraine, among others, closer to the EU — it found wide popular support in the country's west, an opinion poll showed. But many in the east preferred a customs union with Russia. Moscow was promoting this union at the time, and was watching with concern as Ukraine prepared to ink agreements with the EU.
Yanukovych backed out of the EU arrangement at the last minute, in favour of a backdoor deal with Putin. The U-turn sparked the Euromaidan movement that brought hundreds of thousands of protesters in Kyiv's central square.
The movement, which became known as the Revolution of Dignity in Ukraine, toppled Yanukovych, who fled to Russia on February 21 2014.
Less than a week later, armed men in green uniforms without insignia seized the Crimean parliament, cut Crimea off from Ukraine, switched off its access to Ukrainian media and, amid an intense propaganda campaign, staged a referendum on the peninsula's reunification with Russia.
Held in March, it allegedly garnered support from 97 per cent of voters. No observers from overseas were present and the vote has not been internationally recognised.
The same spring, Russian-backed separatists in the Donbas region of eastern Ukraine began to demand independence, leading to a war that has so far killed some 14,000 people.
Large-scale fighting has subsided but the region is caught in a stalemate, with two self-proclaimed and Russia-backed separatist republics established around the cities of Donetsk and Luhansk.
Two agreements, brokered by Germany and France via a set of talks known as the Normandy Format, were signed in 2014 and 2015 in Belarus's capital Minsk. These have attempted to lay the foundations for peace, but there has been little progress. Officially, Russia denies it is one of the sides in the conflict and agrees to participate solely as a mediator.
The election of Volodymyr Zelensky as Ukraine's president in 2019 was followed by several prisoner swaps, but relations between Moscow and Kyiv have since soured further.
In spring 2021, Russia moved more than 100,000 troops to the Ukrainian border. The Normandy talks, designed to implement the so-called Minsk agreements, were severely undermined in November when Russia's foreign ministry published its private diplomatic correspondence with Germany and France.
The same month, the US and its allies began warning that Russia may be planning another invasion. Moscow issued a set of security proposals in the form of two treaties, including a request that Ukraine never join Nato and the alliance roll back its presence from former Warsaw pact states.
Putin and US President Joe Biden spoke twice by phone before the new year in an attempt to resolve the crisis, and a string of meetings were held in January between Russian diplomats and their counterparts in the US, Nato and the Organization for Security and Co-operation in Europe. The conflict remains unresolved.
Additional reporting by Georgina McCartney in London
Written by: Polina Ivanova and Roman Olearchyk
© Financial Times
Six people have now died in a suspected mass methanol poisoning event.