This is a contested question — I'll gather Russia's stated justifications and how international law and global bodies have assessed them.
Russia's full-scale invasion of Ukraine launched on 24 February 2022 is widely considered unjustified and illegal under international law, though Russia has offered several justifications for it.
1. Russia's stated justifications
On 23-24 February 2022, President Vladimir Putin announced a "special military operation" in Ukraine. He stated the aims were to:
•"protect people who have been subjected to abuse and genocide" by the Ukrainian government and to "protect Russia and our people"
•carry out the "demilitarisation and denazification of Ukraine"
•defend the Donetsk People's Republic and Luhansk People's Republic, which Russia had recognized as independent states two days earlier, after they had allegedly requested assistance
Additional arguments made by Russian officials and in Russian commentary since then include:
•Protection of Russian speakers / humanitarian intervention: protecting Russian-speaking populations in the Donbas from alleged genocide and bullying
•NATO expansion and security concerns: One of Russia's primary justifications is the eastward expansion of NATO. Putin portrayed NATO's eastward enlargement as an existential threat to Russia's security and argued the alliance was encroaching on Russia's borders. Russian leaders framed Ukraine's potential NATO membership as crossing a "red line". Another component was that NATO has grown eastward since the 1990s.
•Historical and legal claims: Russia argued its use of force was lawful under Article 51 of the UN Charter on collective self-defense to defend Donetsk and Luhansk, and compared its actions to interventions by the United States and allies in Kosovo, Iraq, Libya and Syria.
Ukraine was not a NATO member at the time of the invasion and is not one today.
2. How these justifications are assessed under international law
The 1945 UN Charter sets out when states may legally use armed force, referred to as jus ad bellum.
•Article 2(4) 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". Along similar lines, Article 2(3) requires states to settle disputes by peaceful means.
•The only two exceptions enshrined in the Charter are self-defense and authorization by the Security Council.
Legal assessment has largely rejected Russia's arguments:
•Since the beginning, the invasion has been described as violating international law including the Charter of the United Nations. The invasion can be grasped as illegal precisely because it violated the Charter.
•It is described by governments as in "flagrant breach of the UN Charter" and as "illegal, unprovoked, and unjustified".
•Self-defense: Experts argued Russia cannot rely on self-defense because Ukraine has not threatened or attacked any other nation, and because Donetsk and Luhansk are not recognized as separate states under international law, so collective self-defense under Article 51 cannot apply.
•Genocide / humanitarian intervention: Scholars described Putin's allegations of genocide and comparison of Ukraine to a Nazi state as baseless. Experts have rejected the humanitarian justification, noting it is widely perceived as a pretext and is unsubstantiated by any evidence that Ukraine has committed acts against Russians in Donetsk and Luhansk that could amount to genocide. Commentators have also questioned whether international law even allows use of force to remedy genocide because the legality of humanitarian intervention is heavily disputed.
•Comparisons to Western interventions: These comparisons have been dismissed as irrelevant because one illegal act does not make a different act legal. Professors have remarked "even if [they were] true, one illegal use of force does not justify another".
The invasion was also said to violate other agreements Russia is party to, including:
•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 and the Minsk agreements
Because it violates the Charter and is more than a minor border incursion, legal experts qualified it as a crime of aggression under Article 8bis(1) of the Rome Statute.
3. Rulings and votes at international bodies
•International Court of Justice (ICJ): Ukraine filed a case arguing Russia's claim of genocide was unfounded. The ICJ ruled in favor of Ukraine and ordered Russia to immediately suspend its military operations, stating there was no evidence to support Russia's claim that Ukraine was committing genocide in eastern regions. The order is legally binding but the ICJ lacks enforcement mechanisms.
•UN Security Council: Russia vetoed a resolution calling for it to immediately cease its attack.
•UN General Assembly: On 2 March 2022 the General Assembly overwhelmingly adopted a resolution that "demands" Russia "immediately" withdraw from Ukraine. The vote was 141-5 with 35 abstentions, described as a powerful rebuke by a vast majority of the world's nations. A series of further resolutions condemning the aggression followed.
4. Distinction between explanation and legal justification
Some analysts distinguish between explaining the war's causes and legally justifying it. They argue NATO expansion is the underlying cause or provocation but note this does not necessarily mean the invasion is morally just or legal under international law. The counterargument, common among Western governments and many legal scholars, is that Ukraine is a sovereign state that may choose its own alliances, and that Russia uses NATO as an excuse for imperialism.
In summary, under the dominant interpretation of international law — as reflected in the UN Charter, ICJ provisional measures, and General Assembly votes — the invasion lacks legal justification. Russia's own stated security and humanitarian rationales are not accepted as lawful grounds for the use of force by the overwhelming majority of states and international legal experts, though Russia continues to assert them as legitimate reasons.