Russia’s war in Ukraine has killed thousands of people, reduced entire cities to rubble and forced millions to flee their homes. The largest military conflict in Europe since World War II has also upset the international security order and sent dangerous ripples through the global economy

In early 2021, a buildup of Russian troops near Ukraine raised fears of an offensive. Moscow withdrew some of the forces in April, paving the way for a June summit between President Joe Biden and Russian President Vladimir Putin. Their meeting failed to meaningfully ease Russia-U.S. tensions, however.

A renewed buildup of Russian troops along Ukrainian borders began in late October and reached an estimated 150,000 troops by the year’s end. From the beginning of the troop surge, Moscow denied any plans to attack Ukraine, calling such Western concerns part of a campaign to discredit Russia. At the same time, it urged the U.S. and its allies to keep Ukraine from joining NATO and roll back the alliance forces from Eastern Europe, demands the West rejected as non-starters.

Then on Feb. 21, Putin abruptly upped the ante, recognizing the independence of pro-Russia rebel regions in eastern Ukraine. Insurgents have been fighting Ukrainian forces there since 2014, when Ukraine’s Moscow-friendly president was driven from office by mass protests and Russia responded by annexing the Crimean Peninsula.

In a televised address on Feb. 24, Putin announced the launch of what he called a “special military operation” intended to demilitarize Ukraine and uproot alleged “neo-Nazi nationalists.” As he spoke, the Russian military unleashed a series of air raids and missile strikes on Ukraine’s military facilities and key infrastructure. Russian troops rolled into Ukraine from Crimea in the south, all along the eastern border and from Moscow’s ally Belarus, which borders Ukraine from the north.

Putin argued that Russia had no choice but to act after Washington and its allies ignored its demand for security guarantees. Western leaders dismissed the claims as a false pretext for the attack.

The Russian military advanced on the Ukrainian capital, Kyiv, located just 75 kilometers (47 miles) south of the border with Belarus, closed in on Ukraine’s second-largest city of Kharkiv in the east and pushed along the Sea of Azov and the Black Sea coasts in the south.

While Russia claimed it was only targeting military facilities, air raids and artillery strikes hit residential areas, schools, and hospitals across Ukraine.