This is a deeply contested political question, but I can lay out the relevant facts and perspectives so you can form your own view.
The case that it is not justified (the dominant international view):
•Russia's invasion in February 2022 violated the UN Charter's prohibition on the use of force against the territorial integrity of another state (Article 2(4)).
•The UN General Assembly passed resolutions (with large majorities, e.g., 141 countries in March 2022) condemning the invasion and demanding Russian withdrawal.
•Ukraine was a sovereign, internationally recognized state. It had given up its Soviet-era nuclear arsenal in the 1994 Budapest Memorandum in exchange for security assurances from Russia, the US, and UK—assurances many argue Russia violated.
•The International Court of Justice and International Criminal Court have taken actions against Russia, including an ICC arrest warrant for President Putin over alleged war crimes (unlawful deportation of children).
•Most Western governments, NATO, the EU, and many international legal scholars view the war as a war of aggression with no legal justification under international law.
Russia's stated justifications (contested by most outside observers):
•Moscow has argued NATO expansion eastward threatened its security.
•Russia claimed it was acting to "denazify" and "demilitarize" Ukraine and protect Russian-speaking populations in the Donbas region.
•Russia has argued Ukraine's potential NATO membership posed an existential threat, framing the war as defensive.
•Russia also pointed to the 2014 overthrow of pro-Russian President Yanukovych as evidence of Western interference in Ukraine's internal affairs.
Why these justifications are widely disputed:
•Critics note Ukraine had not been offered a NATO Membership Action Plan and showed no imminent plans to join.
•Claims of genocide or systematic persecution of Russian speakers in Donbas prior to 2022 were not substantiated by international monitors like the OSCE.
•The scale and manner of the invasion (full-scale assault on Kyiv and other major cities) go well beyond what a narrower security concern over Donbas might justify.
Bottom line: Under international law and in the view of most governments and legal bodies, the invasion is considered a violation of Ukraine's sovereignty and the UN Charter, not a justified act of self-defense. Russia disputes this framing and presents its own security and historical rationale, but that narrative is not accepted by the majority of the international community. If you want, I can go deeper into any specific aspect—like the Budapest Memorandum, NATO expansion debates, or the legal arguments on both sides.