WASHINGTON — All eyes are set on Eastern Europe as Russia begins what has been described as a "full-scale invasion" of Ukraine by the country's foreign minister.
In Kyiv, thousands of residents have packed up their things and evacuated west to avoid further danger, with reports of Russian troops having taken over Antonov Airport, just 15 miles outside the capital.
But why exactly is Russia so keen on invading a sovereign nation? What sparked this years-long conflict to culminate into such a dramatic takeover?
There is not one simple answer to these questions, but rather a succession of events and brewing tension since before the collapse of the Soviet Union. And one alliance, NATO, is at the forefront of this conflict.
Here's a quick summary to catch you up to speed on the Russia-Ukraine crisis:
What led to Russia invading Ukraine?
It is important first to understand the history of Russia and its former Soviet allies.
Following World War II, the U.S. and Western European forces formed the North Atlantic Treaty Organization, an intergovernmental military alliance initially created to blockade the Soviet Union's growing sphere of communist influence.
Most importantly, NATO members agree to Article 5 of the treaty, which states that an attack on any NATO member is considered an attack on all members. This was enough to deter the Soviet Union from ever invading countries who had joined NATO.
Starting in 1999 -- 8 years after the collapse of the Soviet Union -- NATO began expanding its boundaries, and by 2020 had added an additional 14 members, many of them once Soviet-controlled countries.
Experts say that Vladimir Putin has seen this as a western provocation to limit post-Soviet Russian influence, with the country's geographic buffer against NATO shrinking considerably.
Ukraine -- Europe's largest country that shares a sizeable land border with Russia -- has long voiced its intention to join NATO, though many of these plans were shelved as political destabilization plagued Ukraine for much of the last decade.
But at a 2021 NATO conference, the member nations reiterated their intention to acquire Ukraine, triggering the most recent Russian military aggression that has accumulated into this invasion.
Was the invasion provoked?
Putin has said that the U.S. and its allies promised to refrain from expanding NATO after the collapse of the Soviet Union, citing it as his primary reason for military escalation after western leaders refused to dismiss Ukraine's entry into NATO.
But this claim is not entirely accurate, according to many U.S. diplomats and former Soviet President Mikhail Gorbachev. The U.S. did float the idea to Russia that it would never dramatically expand NATO, but this was never a formal agreement.
Russia's feelings that they were tricked after the fall of the Soviet Union is where a sense of betrayal comes from that has tarnished Russia's foreign relations with the West.
Relations were further complicated after Russia annexed Crimea -- a strategic military and shipping base of the Black Sea -- in 2014. The peninsula has been historically tied to Russia, but it had been integrated into major economic asset for Ukraine after the fall of the Soviet Union. NATO does not recognize Russia's sovereignty over Crimea.
Following the annexation, several ethnically-Russian factions in eastern Ukraine also took to arms, establishing two pro-Russian separatist territories. Since then, the United States has only become more involved in the conflict, now providing about 90% of all military aid to Ukraine and furthering angering Putin.
What does Russia want with Ukraine?
The most obvious reason for invasion is for Russia to prevent Ukraine from joining NATO. NATO's land border with Russia would significantly increase if Ukraine had joined.
Moreover, Russia established a naval base in Crimea after its annexation. If Ukraine were to join NATO, this would give Ukraine significantly more resources to potentially take back the peninsula.
But Putin has floated other justifications and outcomes for the military escalation.
For one, Putin sees Ukrainians and Russians as one people, and he says the invasion is meant to protect the citizens of eastern Ukraine, many of whom reside in the pro-Russian separatist areas of the country.
Putin has also said that he intends to "denazify" the country. Russia has a strong identity tied to its catastrophic blow during WWII, and the Russian government has tried to link Nazism to Ukrainian nationalism since the topple of the pro-Russian government in 2014. Ukraine is now led by a Jewish president who lost relatives in the Holocaust and angrily dismissed those claims.