This question involves contested claims about the war — I'll gather Russia's stated reasons and the international legal assessments to give you a balanced overview.
Russia's full-scale invasion of Ukraine, which began on February 24, 2022, is described by Russia as a "special military operation". Whether it is "justified" depends on the legal and political framework used to evaluate it. Russia's stated justifications and the assessment by most of the international community differ sharply:
1. Russia's Stated Justifications
Russian President Vladimir Putin stated that the overarching aims were to "deNazify" and "demilitarize" Ukraine.
In his announcement of the invasion, other claims included:
•Threat from NATO expansion: Russia asserted that its national interests and "very existence" were under threat from the eastward expansion of the North Atlantic Treaty Organization (NATO), culminating in the claim that there was "no other way to defend Russia," thereby characterizing the military action as an exercise of the right to self-defense. He also claimed that NATO was building up military infrastructure in Ukraine, threatening Russia.
•Protection of Russian speakers in Donbas: Putin claimed that Ukraine had been committing genocide against Russian speakers in the region and that Russian-speaking populations in Donetsk/Luhansk needed protection.
•Collective self-defense and humanitarian intervention: Asserting that Donetsk and Luhansk were being repressed and subject to genocide by the Ukrainian government since 2014, Putin justified his use of force as one of collective self-defense on behalf of the country's east. Russia also attempted to justify its actions under the doctrine of humanitarian intervention and invoked UN Charter Article 51 on self-defense, and claimed Russia had no plans to occupy Ukrainian land.
2. Assessment Under International Law and by International Bodies
Most international lawyers, the United Nations, and human rights organizations have assessed the invasion as a violation of international law:
•UN Charter Article 2(4): The invasion is described as a quite blatant violation of Article 2(4) of the UN charter, which provides that states are not allowed to use force in their international relations. That article prohibits the "threat or use of force against the territorial integrity or political independence of any state".
•No valid self-defense claim: Article 51 of the UN Charter permits self-defence only in limited circumstances, and self-defense is the only justification for use of force against another country according to international law. Analyses concluded neither of these exceptions applied to Russia's invasion. Ukraine did not launch an armed attack against Russia, and the Security Council did not authorize any use of force. Therefore, the suggestion that Russia's use of force is justified under Article 51 has no support in fact or law.
•Overall legality: One analysis concluded: Under no scenario is Russia's armed invasion of Ukraine legal under contemporary international law and norms.
•Genocide claim: The allegations that Ukraine was committing genocide were widely rejected as untrue. A few weeks into the war, ICJ judges said they hadn't seen evidence to corroborate Russia's genocide claim. The International Court of Justice (ICJ) in March 2022 ordered provisional measures directing Russia to suspend military operations, by 13 votes to 2.
•Characterization as aggression: Amnesty International said Russia's invasion of Ukraine is a clear violation of the United Nations Charter and an act of aggression that is a crime under international law.
3. United Nations Response
•The United Nations General Assembly approved a nonbinding resolution condemning Russia for invading Ukraine and demanding that it withdraw its military forces.
•The vote on March 2, 2022 was 141-5 with 35 abstentions, with 141 of the 193 members of the assembly voting in favor during a rare emergency session. Only Belarus, Eritrea, North Korea and Syria voted with Russia against the resolution. A later resolution in October 2022 condemning Russia's attempted annexation of Ukrainian territory also passed overwhelmingly.
Summary
From Russia's perspective, the invasion was presented as a necessary defensive and protective action against NATO encroachment and alleged mistreatment of Russian speakers in eastern Ukraine.
From the perspective of the UN Charter, the UN General Assembly majority, the International Court of Justice, and the prevailing view among international law scholars, the invasion violated the prohibition on the use of force, did not meet the criteria for self-defense or Security Council authorization, and was not supported by evidence for the genocide claim, and is therefore considered illegal and unjustified under international law.