This is a contested legal and historical question — I'll gather Russia's stated justifications and the international response to assess them.
Russia's 2022 invasion of Ukraine is presented differently depending on whether the justification is assessed under Russian official claims or under international law.
1. Russia's stated justifications
On 23-24 February 2022, President Vladimir Putin announced a "special military operation" in Ukraine. He claimed that:
•military intervention was necessary to "protect people who have been subjected to abuse and genocide" by the Ukrainian government and to "protect Russia and our people", and that the Donetsk and Luhansk People's Republics — which Russia had recognized two days earlier — had requested assistance in their fight against the Ukrainian government.
•The stated aims included "the demilitarisation and denazification of Ukraine".
Other arguments raised by Russian officials include:
•Collective self-defense: Russia argued its use of force was lawful under Article 51 of the UN Charter, which preserves rights to defend against "an armed attack" and to engage in "collective self-defense," specifically to defend the Donetsk and Luhansk People's Republics.
•Humanitarian protection / prevention of genocide in the Donbas.
•Security concerns related to NATO expansion and closer military ties between Ukraine and Western militaries. Moscow's perceived "red lines" included the 2008 Bucharest declaration on NATO membership, and Moscow framed the invasion as a preemptive military necessity to halt loss of strategic depth and prevent Western power projection.
•Historical comparison: Russia compared its actions to interventions by the United States and allies in Kosovo, Iraq, Libya and Syria.
Scholars described Putin's allegations of genocide and comparison of Ukraine to a Nazi state as baseless.
2. Assessment under international law
The prevailing assessment among international lawyers, foreign affairs experts, and UN bodies is that the invasion violates international law.
•Since the beginning, the invasion has been described as violating international law including the Charter of the United Nations. Under no scenario is Russia's armed invasion of Ukraine legal under contemporary international law and norms.
The legal framework is:
•Article 2(4) of the UN Charter provides that all members "shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state". Article 2(3) requires states to settle disputes by peaceful means.
•The only two exceptions enshrined in the Charter are self-defense and an authorization by the Security Council.
Experts have generally rejected Russia's legal justifications:
•Self-defense: Many experts opined the invasion violated Article 2(4)'s prohibition on use of force and rejected Russia's official legal justifications. Experts such as John B. Bellinger III, Gabriella Blum, Naz Modirzadeh and Anthony Dworkin criticized the Article 51 argument, arguing Russia cannot rely on self-defense because Ukraine has not threatened or attacked any other nation, and that even if Ukraine had planned an attack on Donetsk or Luhansk, Russia could not invoke collective self-defense because these regions are not recognized as separate states under international law.
•Humanitarian / genocide justification: Experts have rejected the argument that the invasion is justified on humanitarian grounds to protect Russian-speakers. The humanitarian justifications are widely perceived as a pretext, and are unsubstantiated by any evidence that Ukraine has committed acts against Russians that could amount to genocide. Some commentators questioned whether international law even allows use of force to remedy genocide because the legality of humanitarian intervention is heavily disputed.
•Comparison to Western interventions: These comparisons have been dismissed as irrelevant because one illegal act does not make a different act legal. Professors Blum and Modirzadeh remarked that "even if [they were] true, one illegal use of force does not justify another".
Additional legal violations cited include:
•Violation of the 1994 Budapest Memorandum, in which Russia, the United States and the United Kingdom agreed "to respect the independence and sovereignty and the existing borders of Ukraine", the 1975 Helsinki Final Act on territorial integrity, and the Minsk agreements.
•Qualification as a crime of aggression under Article 8bis(1) of the Rome Statute, defined as an act of aggression which by its character, gravity and scale constitutes a manifest violation of the Charter.
3. Response at the United Nations
•Article 2(4)'s prohibition is central, and the invasion has also been called a crime of aggression under international criminal law.
•A UN Security Council resolution calling for Russia to cease its attack was vetoed by Russia.
•The UN General Assembly adopted Resolution ES-11/1 on 2 March 2022. It deplored Russia's invasion of Ukraine and demanded a full withdrawal of Russian forces and a reversal of its decision to recognise the self-declared People's Republics of Donetsk and Luhansk.
•The resolution passed by 141-5 with 35 abstentions, also described as an overwhelming 141-5 vote majority with 35 nations abstaining. The five states voting against were Russia, Belarus, Syria, North Korea and Eritrea.
Summary
•From Russia's perspective, the invasion is justified as self-defense, protection of Russian speakers from alleged genocide, denazification/demilitarization, and as a response to NATO-related security threats.
•From the perspective of international law as interpreted by the UN Charter, the UN General Assembly, the International Court of Justice, and the majority of international legal scholars, the invasion violates Article 2(4) of the UN Charter, lacks valid self-defense or Security Council authorization, is not supported by evidence of genocide, and has been condemned as illegal aggression.
Whether the invasion is considered "justified" therefore depends on the framework applied — political/historical claims made by Russia versus the legal standards of the UN Charter and international humanitarian law, under which it is widely deemed illegal and unjustified.