This question involves contested legal and political claims - I'll check the stated justifications and how they hold up under international law.
Russia's invasion of Ukraine launched on 24 February 2022 is widely considered illegal under international law, though Russia has offered several legal and political justifications for it.
1. Russia's Stated Justifications
In his televised address on 23-24 February 2022 announcing a "special military operation," President Vladimir Putin said 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". The stated aims included "the demilitarisation and denazification of Ukraine".
Other components of the Russian justification include:
•Protection of ethnic Russians and Russian-speakers: The primary reason cited by Russia was to counter what it claimed was Ukrainian aggression and to protect ethnic Russians living in Ukraine. Related claims included allegations of genocide against pro-Russian individuals in eastern Ukraine.
•Self-defense and collective self-defense: Russia has argued that its use of force is 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 People's Republic and Luhansk People's Republic, which Russia recognized as independent states two days before the invasion.
•NATO expansion as a security threat: Putin stated that the United States could deploy missiles in Ukraine which could threaten Russian nuclear deterrence, and declared that this was one of the principal reasons for the invasion. Before the invasion, Russia demanded that the United States and Ukraine formally renounce NATO accession for Ukraine, and that concern over NATO was a primary driver of the decision. In his speech, Putin also pointed to what he described as Western violations of international law in Kosovo, Iraq, and Libya as precedent.
Scholars have described Putin's allegations of genocide and comparison of Ukraine to a Nazi state as baseless.
2. Assessment Under International Law
The UN Charter sets out the conditions under which UN members may legally resort to force.
•Prohibition on use of force: 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". The only two exceptions enshrined in the Charter are self-defense and authorization by the Security Council.
•View of the invasion as a violation: Russia's invasion of Ukraine is described as a clear act of aggression and a manifest violation of Article 2.4. Since the beginning, the invasion has been described as violating international law including the Charter of the United Nations.
•Rejection of self-defense claim: Experts have criticized the self-defense argument. Arguments that Russia cannot rely on self-defense because Ukraine has not threatened or attacked any other nation are made by legal scholars. They also argue that even if Ukraine had been planning an attack against Donetsk or Luhansk, Russia could not invoke collective self-defense because these regions are not recognized as separate states under international law. One analysis notes the claims are "nonsense. Neither Ukraine nor NATO had launched or was about to launch any such attack. And, as the enclaves are not independent states ... they cannot ask another state to defend them".
•Rejection of humanitarian/genocide justification: Experts have rejected the argument that the invasion is justified on humanitarian grounds to protect Russian-speakers in the Donbas, noting that whether international law even allows use of force to remedy genocide or human rights violations is heavily disputed. Russia's humanitarian justifications are widely perceived as a pretext and unsubstantiated by evidence that Ukraine has committed acts amounting to genocide.
•Comparison to Western interventions: Russia's comparisons to interventions by the United States and allies in Kosovo, Iraq, Libya and Syria have been dismissed as irrelevant because "one illegal act does not make a different act legal".
3. Position of the United Nations and International Bodies
•The UN Security Council was blocked from demanding Russia cease its attack because Russia vetoed the resolution.
•The UN General Assembly overwhelmingly adopted a resolution demanding the Russian Federation immediately end its invasion of Ukraine and unconditionally withdraw all its military forces. The resolution passed on 2 March 2022 (ES-11/1) with 141 in favor and 5 against, with the Assembly deploring "in the strongest terms the aggression by the Russian Federation against Ukraine in violation of Article 2 (4) of the Charter".
•The invasion is also described as violating other agreements Russia is party to, including the 1975 Helsinki Final Act, the 1994 Budapest Memorandum in which Russia agreed to respect Ukraine's existing borders, and the Minsk agreements.
In legal terms, because it violates the UN Charter and is more than a minor border incursion, experts have qualified the intervention as a crime of aggression under Article 8bis of the Rome Statute.
In summary, under the dominant interpretation of the UN Charter and international law adopted by the UN General Assembly, the International Court of Justice, and most international legal scholars, the invasion is not considered legally justified. Russia maintains that it is justified on grounds of self-defense, protection of populations in Donbas, and preempting a NATO threat.