CNN  — 

Russia’s multi-pronged invasion of Ukraine has thrust the country into a conflict that many on the European continent had thought was one for the history books. Now the country is in the throes of war, with a humanitarian disaster unfolding.

After months of military buildup and brinkmanship on Russia’s side of the border, Ukraine’s 44 million residents woke up to an all-out conflict on Thursday. Fighting has erupted in several cities across the country, including in the capital, Kyiv, and nearly a half a million people have fled to neighboring countries, according to the United Nations.

Russia had been tightening its military grip around Ukraine since last year, amassing tens of thousands of soldiers, as well as equipment and artillery, on the country’s doorstep.

Black smoke rises from a military airport in Chuguyev near Kharkiv, the second-largest city in Ukraine,  on Thursday.

Frenzied diplomatic efforts early this year failed to avert the worst-case scenario. Now those troops are engaged in combat with Ukrainians for control of the country.

The escalation in a years-long conflict between the nations has now triggered the greatest security crisis in Europe since the Cold War. Russia’s attack on the country has also sparked an intense showdown between Western powers and Moscow.

So how did we get here? The picture on the ground is shifting rapidly, but here’s a breakdown of what we know.

How did Russia invade Ukraine?

Several areas across Ukraine came under attack on Thursday morning after Russian President Vladimir Putin declared the start of a “special military operation” and warned of bloodshed unless Ukrainian forces lay down their arms.

The move came after months of speculation about what Moscow’s intentions were with the troops it had massed on the Ukrainian border. At least 150,000 Russian troops encircled the country on three sides, like a sickle, according to estimates from US and Ukrainian intelligence officials.

In late 2021 and early 2022, fears heightened as satellite images revealed new Russian deployments of troops, tanks, artillery and other equipment cropping up in multiple locations, including near eastern Ukraine, Crimea and Belarus, where its forces were participating in joint drills with Moscow’s closest international ally.

Some of those forces began pouring across the border, crossing into Ukraine from the north in Belarus and to the south from Crimea, according to the Ukrainian State Border Service. Elsewhere, explosions rang out in multiple cities, including the capital Kyiv.

Missile strikes and street fighting have raged in the days since. Military aged men have been ordered to stay in Ukraine, while countless others have fled westwards towards Poland or Romania.

Russia’s larger and far better-equipped military has, faced determined resistance across the country, as ordinary Ukrainians and reservists have joined efforts to defend their families and homes, frustrating Moscow’s attempts.

That resistance has been “stiffer than expected” and Russia has had unexpected difficulties supplying its forces, two senior US officials with direct knowledge told CNN. On the battlefield, Russia is suffering heavier losses in personnel and armor and aircraft than expected. This is due in part to the fact that Ukrainian air defenses have performed better than pre-invasion US intelligence assessments had anticipated.

But US intelligence and defense officials closely tracking the Russian campaign say that Putin still holds a number of moves in reserve that could devastate the Ukrainian resistance.

The US and its allies have said they have no intention of sending troops into Ukraine, which is not a NATO member. But Ukraine has received assistance in other forms from Europe, the US and beyond, as the West united in condemnation of Putin’s move. NATO’s Secretary General, Jens Stoltenberg condemned the Russian attack as a “grave breach of international law, and a serious threat to Euro-Atlantic security.”

And a raft of heavy sanctions have threatened to cripple Russia’s economy; Moscow has been virtually cut off from the Western financial apparatus and the value of its currency, the ruble, has tanked.

The coordinated assault came days after Putin announced that Moscow would officially recognize the self-declared Donetsk and Luhansk People’s Republics (DNR and LNR), in eastern Ukraine’s Donbas region, ordering the deployment of Russian troops there in what was widely believed to be the opening salvo to a broader military confrontation.

The territory recognized by Putin extended beyond the areas controlled by pro-Russian separatists, raising red flags about Russia’s intended creep into Ukraine.

What set the stage for the conflict?

Ukraine was a cornerstone of the Soviet Union until it voted overwhelmingly for independence in a democratic referendum in 1991, a milestone that turned out to be a death knell for the failing superpower.

After the collapse of the Soviet Union, NATO pushed eastward, bringing into the fold most of the Eastern European nations that had been in the Communist orbit. In 2004, NATO added the former Soviet Baltic republics Estonia, Latvia and Lithuania. Four years later, it declared its intention to offer membership to Ukraine some day in the distant future – crossing a red line for Russia.

Putin has seen NATO’s expansion as an existential threat, and the prospect of Ukraine joining the Western military alliance a “hostile act” – a view he invoked in a televised speech on Thursday, saying that Ukraine’s aspiration to join the military alliance was a dire threat to Russia.

In interviews and speeches, Putin has previously emphasized his view that Ukraine is part of Russia, culturally, linguistically and politically. While some of the mostly Russian-speaking population in Ukraine’s east feel the same, a more nationalist, Ukrainian-speaking population in the west has historically supported greater integration with Europe.