This question involves contested claims about legality and justification — I'll gather Russia's stated reasons and the international legal assessment to give you a balanced overview.
Russia's full-scale invasion of Ukraine, launched on 24 February 2022, is presented differently by Russia and by the overwhelming majority of the international community.
1. Russia's stated justifications
When announcing what Russia calls a "special military operation," President Vladimir Putin stated that the overarching aims were to "deNazify" and "demilitarize" Ukraine.
In a televised address on 23 February, Putin claimed that Russian military intervention in Ukraine was necessary to "protect people who have been subjected to abuse and genocide" by the Ukrainian government and to "protect Russia and our people".
Other stated rationales that have been repeated by Russian officials include:
•Acting to prevent a genocide in eastern Ukraine - Ukraine filed a case with the International Court of Justice (ICJ) shortly after the invasion began, saying that Moscow's stated justification, that it was acting to prevent a genocide in eastern Ukraine, was unfounded.
•Protection of Russian-speaking populations in the Donbas and collective self-defense of the self-proclaimed Donetsk and Luhansk republics. Russia claimed it was acting under Article 51 to protect Russianspeaking populations in the Donbas and to engage in collective selfdefense of the selfproclaimed Donetsk and Luhansk republics.
•Countering a threat from NATO expansion and Western influence over Ukraine. Moscow continues to describe the war as a defensive campaign against what it calls an existential NATO-backed threat, with Article 51 of the UN Charter as the legal umbrella.
Putin's reference to Article 51 of the UN Charter is regarded by a number of lawyers as incorrect - Article 51 allowed one UN member state to give military aid to another member state; but the DPR and LPR were not UN member states and were only recognized as independent by Russia.
2. Position under international law
The 1945 UN Charter sets out the conditions under which UN member states may legally resort to war or the use of armed force in general. The central rules are:
•Article 2(4) states, “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".
•Self-defense is the only justification for use of force against another country, according to international law. This condition is found in the U.N. Charter and is binding for all 193 U.N. member countries, absent Security Council authorization under Article 42.
Legal analyses widely conclude:
•Under no scenario is Russia’s armed invasion of Ukraine legal under contemporary international law and norms.
•The Russian invasion of Ukraine is a brutal murder of the United Nations (UN) Charter and several other tenets of international law.
•There is no legal justification under Article 51 of the UN Charter for the military actions Russia has taken against Ukraine. There being no basis to claim self-defense, the actions by the Russian military represents an illegal aggression.
•The 2022 Russian invasion can be grasped as illegal precisely because it violated the Charter – arguably one of the most significant documents of international law.
•Even if Putin's primary concern is NATO, this does not justify invasion. The UN Charter and customary international law prohibit unilateral use of force except in self-defense (Article 51), which Russia has not credibly invoked.
Russia's actions have also been qualified by legal experts as a crime of aggression under Article 8bis(1) of the Rome Statute, which defines it as "an act of aggression which, by its character, gravity and scale, constitutes a manifest violation of the Charter of the United Nations." This includes the "invasion or attack by the armed forces of a State of the territory of" another State.
3. Response of international institutions
•UN General Assembly: On 2 March 2022, during the 11th emergency special session, the UN General Assembly adopted resolution (ES-11/1) demanding that Russia immediately ends its military operations in Ukraine. The resolution stated that it "Deplores in the strongest terms the aggression by the Russian Federation against Ukraine in violation of Article 2 (4) of the Charter". The vote was 141 in favor, 5 against, with 35 abstentions. A later summary describes that the General Assembly deplored "in the strongest terms the aggression by the Russian Federation against Ukraine in violation of Article 2 (4) of the Charter".
The EU and other states have reiterated condemnation of Russia's "illegal, unprovoked and unjustified war of aggression, which constitutes a manifest violation of international law, including the UN Charter".
•International Court of Justice (ICJ): On 16 March 2022, the ICJ ruled by a vote of 13 to two that Russia "shall immediately suspend the military operations that it commenced on 24 February". The Court's press release stated: "The Russian Federation shall immediately suspend the military operations that it commenced on 24 February 2022 in the territory of Ukraine".
4. Summary
•From Russia's perspective, the invasion is justified as a defensive and protective action for Russian speakers, to prevent alleged genocide, to demilitarize and denazify Ukraine, and to counter NATO expansion.
•From the perspective of the UN Charter, the UN General Assembly, the ICJ, and the majority of international law scholars, those justifications do not meet the legal criteria for self-defense or Security Council authorization, and the invasion is therefore considered illegal and an act of aggression.
Whether an action is "justified" depends on the framework used - political and security arguments invoked by Russia versus the legal framework of the UN Charter and international law that most states and courts apply.