This is a contested question with competing legal and political claims — I'll gather the stated justifications and how international law assesses them.
Russia's invasion of Ukraine, launched February 24, 2022, is presented differently by Russia and by the overwhelming majority of international legal bodies.
Russia's Stated Justifications
On February 23-24, 2022, President Vladimir Putin announced a "special military operation" and 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". The stated aims included "the demilitarisation and denazification of Ukraine".
Other justifications offered by Russia include:
•Collective self-defense under Article 51 of the UN Charter: Russia claimed it could use force to defend the Donetsk People's Republic and Luhansk People's Republic, which it recognized as independent states two days before the invasion.
•Protection of Russian speakers: Russia claims a right to use force to protect people beyond its borders who speak its language or share its culture.
•Security concerns and NATO expansion: Russia views NATO expansion as an existential threat, and has stated that Ukraine joining NATO is a red line. A related claim is that its security interests justify its war.
•Illegitimacy of Ukrainian government: Russia claims that Ukraine's democratically elected government is illegitimate.
•Pre-emptive self-defense: Russia claims that it acted pre-emptively, in self-defence.
Analysts also describe broader political motivations, including concern that Ukraine was moving out of Moscow's orbit and a desire to reassert a sphere of influence.
Assessment Under International Law
Since the beginning, the Russian invasion of Ukraine has been described as violating international law (including the Charter of the United Nations).
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 their international disputes by peaceful means".
•The only two exceptions enshrined in the UN Charter are self-defense and an authorization by the Security Council.
•According to Article 2.4 all Members shall refrain from the threat or use of force against the territorial integrity or political independence of any State, with the only exceptions being self-defence and where such use of force is authorised by the UN Security Council.
•Experts state that "Under no scenario is Russia's armed invasion of Ukraine legal under contemporary international law and norms", and that self-defense is the only justification for use of force against another country according to international law.
Responses to Russia's specific legal arguments:
1.Self-defense / Collective Self-Defense: Experts have criticized this argument and argued that Russia cannot rely on a self-defense justification because Ukraine has not threatened or attacked any other nation. 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.
2.Genocide / Humanitarian Intervention: Scholars described Putin's allegations of genocide and comparison of Ukraine to a Nazi state as baseless. Experts have rejected Russia's argument that its invasion is justified on humanitarian grounds to protect Russian-speakers in the Donbas. Russia's humanitarian justifications are widely perceived as a pretext, and are unsubstantiated by any evidence that Ukraine has committed acts that could amount to genocide. A right to strike first against threats that a state alone perceives would leave every state exposed to the fears of its neighbours, real or invented.
3.Violation of Other Agreements: The invasion is cited as violating the 1975 Helsinki Final Act, in which the USSR promised not to violate the "territorial integrity" of other signees, and the 1994 Budapest Memorandum, in which Russia agreed "to respect the independence and sovereignty and the existing borders of Ukraine".
Response of International Bodies
•The invasion has also been called a crime of aggression under international criminal law, defined as an act which by its character, gravity and scale, constitutes a manifest violation of the Charter of the United Nations.
•At the UN Security Council, Russia vetoed a resolution calling for it to immediately cease its attack.
•At the UN General Assembly, a resolution condemning the Russian invasion was passed with an overwhelming 141-5 vote majority, with 35 nations abstaining.
•The UK statement to the OSCE summarized the international critique that Russia has never offered a single consistent explanation, but has offered a succession of justifications, each falsely presented as a matter of principle, and argued that a rule that a state claims for itself, but would never accept being applied to itself, is not a principle.
In summary, while Russia asserts security, humanitarian, and historical reasons for its actions, the prevailing view among international courts, UN bodies, and international law experts is that the invasion violates the UN Charter's prohibition on the use of force, does not meet the criteria for self-defense or authorized humanitarian intervention, and constitutes an illegal act of aggression.