Why is Russia attacking Ukraine? Here are 5 reasons Putin and others have given for the invasion

• Russian President Vladimir Putin launched a large-scale invasion of Ukraine early Thursday morning.
• Putin is concerned over NATO's expansion and has falsely said Ukraine is committing genocide.

Russian forces attacked Ukraine early Thursday morning, launching a large-scale and unprovoked invasion that was feared for weeks.

The Russian military entered Ukraine from several directions, with troops headed toward its capital, Kyiv, and attacked with airstrikes and shelling. Ukrainian authorities said dozens of troops on both sides and some Ukrainian civilians were killed amid the fighting.

President Joe Biden and other Western leaders have criticized Russian President Vladimir Putin, calling his country's actions a violation of international law. On Thursday afternoon, Biden announced new, harsher sanctions against Russia that he said were designed "to maximize the long-term impact on Russia and minimize the impact on the United States and our allies."

Here are some reasons Putin has given for why Russia invaded Ukraine — some of which are based on falsehoods — along with what the US and NATO have said about his motivations.

Concern over NATO's eastward expansion

Putin has expressed concern over the expansion of NATO, the North Atlantic Treaty Organization, into Eastern Europe and former Soviet Republics, especially Ukraine.

NATO is a political and military alliance that dates back to 1949 and consists of 30 member countries, including the US, Canada, the UK, France, Belgium, and many other European nations. NATO membership is open to any European country that meets its criteria. The alliance has said Ukraine is among three nations that are considered aspiring members.

Putin has criticized NATO for expanding eastward since the fall of the Soviet Union in 1991. He has said NATO enlisting nations on Russia's borders represents a provocation, though NATO insists it is a defensive alliance and not a threat to Russia.

In a speech Thursday morning, Putin said the invasion of Ukraine was an act of self-defense against NATO expansion.

Putin baselessly claims Ukraine is committing genocide against ethnic Russians

Putin has also accused Ukraine of committing genocide and called its government a Nazi regime, claims for which there is no evidence.

Earlier this month, Putin claimed genocide was being committed against ethnic Russians in eastern Ukraine, particularly in the Donbas region, where Kremlin-backed rebels have been fighting with Ukrainian forces since 2014.

On Monday, Putin recognized Donetsk and Luhansk, two separatist regions in the Donbas, as independent states and ordered Russian troops in for a "peacekeeping" operation, a move that was widely seen as a pretext for war.

In his Thursday speech announcing the invasion, Putin again repeated the claims of genocide, saying he was seeking the "denazification" of Ukraine, a country whose democratically elected leader, President Volodymyr Zelensky, is Jewish.

Putin falsely says Ukraine isn't a real country

In a lengthy speech Monday, Putin claimed Ukraine wasn't a real country: "Ukraine has never had its own authentic statehood. There has never been a sustainable statehood in Ukraine."

He argued Ukraine was created by the Soviet Union under its first leader, Vladimir Lenin, despite overwhelming evidence of Ukrainian culture and history that predates the Soviet Union. Putin also insisted that Ukraine is part of Russia.

"Let me emphasize once again that Ukraine for us is not just a neighboring country. It is an integral part of our own history, culture, spiritual space," he said.

Putin has also incorrectly stated that Russia gave Ukraine the right to break away, when in fact, the Ukrainian people voted overwhelmingly for independence in a 1991 referendum, The Washington Post reported. For the referendum vote, 84% of eligible voters went to the polls, and more than 90% voted in favor of independence.

Dubious concerns over nuclear weapons

After the collapse of the Soviet Union, Ukraine voluntarily gave up the nuclear weapons in its territory in exchange for a guarantee of security by the US, the UK, and Russia.

Putin, however, has expressed concern that Ukraine has the knowledge and desire to obtain nuclear weapons, thus posing a threat to Moscow. While there is no evidence that Ukraine is trying to acquire nuclear weapons, The New York Times reported that Putin had spewed conspiracy theories to talk up the threat and use it to justify an invasion.

"If Ukraine acquires weapons of mass destruction, the situation in the world and in Europe will drastically change, especially for us, for Russia," Putin said Tuesday. "We cannot but react to this real danger, all the more so since, let me repeat, Ukraine's Western patrons may help it acquire these weapons to create yet another threat to our country."

US officials and NATO have repeatedly said there are no plans to arm Ukraine with nuclear weapons.

Putin may want to build back a Russian empire

Another reason that some say Russia is invading Ukraine is one that Putin has never outright said: to build back an empire and restore the control Russia, or the Soviet Union, had over Europe and Asia during the Cold War.

Biden said as much Thursday in comments on Putin's invasion of Ukraine: "He has much larger ambitions than Ukraine. He wants to, in fact, reestablish the former Soviet Union. That's what this is about."

"I think that his ambitions are completely contrary to the place where the rest of the world has arrived," he added.

The US ambassador to the United Nations made similar comments earlier this week, saying Putin wanted to go back to the days of the Russian Empire, which preceded the Soviet Union.

"That includes all of Ukraine. It includes Finland. It includes Belarus and Georgia and Moldova. Kazakhstan, Kyrgyzstan, and Tajikistan. Turkmenistan, Uzbekistan, and Lithuania. Latvia and Estonia. It includes parts of Poland and Turkey," Ambassador Linda Thomas-Greenfield said at a UN Security Council meeting.

"In essence, Putin wants the world to travel back in time, to a time before the United Nations, to a time when empires ruled the world. But the rest of the world has moved forward. It is not 1919, it is 2022," she added.