That question involves competing legal and historical claims — I'll gather the stated justifications and how international law assesses them.
Russia's full-scale invasion of Ukraine on 24 February 2022 has competing claims about justification:
1. Russia's Stated Justifications
In his televised address announcing a "special military operation," President Vladimir Putin said:
•Russia was launching the operation to defend the Russian-speaking territories in eastern Ukraine — the Donetsk People's Republic and Luhansk People's Republic — under Article 51 of the United Nations Charter, citing the decision of the Federation Council and treaties of friendship with the two republics.
•The purpose was to "protect the people" in Donbas who, according to Putin, "for eight years now, have been facing humiliation and genocide perpetrated by the Kyiv regime" and to pursue "demilitarization and denazification" of Ukraine.
•Other claims included that Ukraine had been committing genocide against Russian speakers, that Ukraine's government were neo-Nazis under Western control, that Ukraine was developing nuclear weapons and that NATO was building up military infrastructure in Ukraine, threatening Russia.
•On NATO, Putin said further expansion of NATO infrastructure and military development in Ukraine are "unacceptable for us", describing NATO expansion eastward as making the situation "worse and more dangerous".
In March 2022, the Russian branch of the International Law Association supported the invasion as being lawful under the self defence provisions of the UN Charter.
2. Assessment Under International Law
The prevailing view among international legal institutions and scholars is that the invasion violates international law:
•UN Charter Article 2(4): Russia's invasion violates Article 2(4) of the UN Charter, which requires member states to refrain from the "use of force against the territorial integrity or political independence of any state".
•Article 51 Self-Defense: Russia invoked Article 51, which provides for the "inherent right of individual or collective self-defense if an armed attack occurs against a member of the United Nations". Analysts note Ukraine did not commit or threaten an armed attack against Russia or any other UN member state, and Donetsk and Luhansk are not UN member states and "do not even qualify as states under international law". Therefore, Article 51 would not permit collective self-defense in this case.
•Genocide Claim: Putin's statements that Ukraine was committing "genocide" were described as not supported by facts and would not give a right to invade even if true. The Genocide Convention defines genocide as certain acts intended to destroy a group, and "There is no evidence that Ukraine engaged in any of the defined actions". The Convention does not authorize use of force to remedy genocide.
•The International Court of Justice (ICJ) in March 2022 ordered Russia to halt its invasion and said it had "not seen any evidence to support the Kremlin's justification for the war, that Ukraine was committing genocide against Russian-speakers". On the ground, Putin's accusation of genocide has been "widely rejected as baseless" and the ICJ said it had not seen any evidence.
•Secession: Russia's recognition of Donetsk and Luhansk as independent states was described as inconsistent with international law governing state sovereignty. While some experts argue remedial secession is permissible as a last resort when a people suffer grave abuses, this is a minority view.
3. United Nations Response
•The UN General Assembly adopted Resolution ES-11/1 on 2 March 2022. It deplored "in the strongest terms the aggression by the Russian Federation against Ukraine in violation of Article 2 (4) of the [UN] Charter".
•The resolution, titled "Aggression against Ukraine," was adopted with 141 votes in favor, 5 against and 35 abstentions, and demanded full withdrawal of Russian forces and reversal of recognition of Donetsk and Luhansk.
In summary, Russia argues the invasion was justified as self-defense, protection of populations in Donbas from alleged genocide, and response to NATO expansion. The United Nations General Assembly, the ICJ, and the majority of international legal experts assess that none of these grounds satisfies the legal exceptions to the prohibition on use of force in the UN Charter, and therefore consider the invasion an act of aggression in violation of Article 2(4).