While tensions on the Ukraine-Russia border have been simmering for a long time, here's a timeline of notable developments over the last decade that culminated into the current escalation in tensions at the Ukraine-Russia border.
2013: Ukraine's former President Viktor Yanukovych was under severe economic pressure from Russia to not join the European Union. And he pulled out of talks with the EU last minute, after a year of insisting that it was intent on signing a historic political and trade agreement that was aimed at creating closer political and economic ties and fostering economic growth among the nations of Eastern Europe and the Caucasus, including Ukraine. This sparked weeks of violent protests in Kyiv.
2014: In March 2014, Russia annexed Crimea, an autonomous peninsula in southern Ukraine with strong Russian loyalties, on the pretext that it was defending its interests and those of Russian-speaking citizens.
2015: Shortly afterwards, pro-Russian separatists in Ukraine's Donetsk and Luhansk regions declared their independence from Kyiv, prompting months of heavy fighting. Despite Kyiv and Moscow signing a peace deal in Minsk in 2015, brokered by France and Germany, there have been repeated ceasefire violations.
According to UN figures, there have been more than 3,000 conflict-related civilian deaths in eastern Ukraine since March 2014.
The European Union and US have imposed a series of measures in response to Russia's actions in Crimea and eastern Ukraine, including economic sanctions targeting individuals, entities and specific sectors of the Russian economy.
The Kremlin accuses Ukraine of stirring up tensions in the country's east and of violating the Minsk ceasefire agreement.
What's happening now: The US and NATO have described the movements and concentrations of troops in and around Ukraine as "unusual."
As many as 100,000 Russian troops have remained amassed at the Ukrainian border. US intelligence findings in December estimated that Russia could begin a military offensive in Ukraine "as soon as early 2022."