Your question touches on contested international claims — I'll gather Russia's stated reasons and the response under international law to give you a balanced overview.
Russia's full-scale invasion of Ukraine, launched on 24 February 2022, is described by Russia as justified and by Ukraine and the vast majority of the international community as unjustified and illegal. The question involves contested political claims and settled international legal principles.
1. Russia's Stated Justifications
On 23-24 February 2022, President Vladimir Putin announced a "special military operation" in Ukraine. Russia gave several overlapping rationales:
•Protection from genocide and abuse: Putin 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". Russian officials have repeated this rationale, including Defense Minister Sergei Shoigu stating Russia is "taking action to save the population from genocide and terrorism".
•Protection of Donetsk and Luhansk: Russia recognized the Donetsk People's Republic and Luhansk People's Republic as independent states two days before the invasion and claimed they had requested assistance in their fight against the Ukrainian government. Russia has argued its use of force is lawful under Article 51 of the UN Charter as collective self-defense of those republics.
•Denazification and demilitarization: Putin stated the overarching aims were to "deNazify" and "demilitarize" Ukraine.
Scholars have described Putin's allegations of genocide and comparison of Ukraine to a Nazi state as baseless.
2. Legal Framework Under the UN Charter
The legal assessment of use of force between states is primarily governed by the 1945 UN Charter.
•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 Charter similarly requires states to "settle their international disputes by peaceful means". The Charter 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".
•Only two exceptions: The only two exceptions enshrined in the UN Charter are self-defense and an authorization by the Security Council. Self-defense is the only justification for use of force against another country according to international law, binding for all 193 UN member countries, and is permitted "if an armed attack occurs" against a country.
Russia's invasion has been described since the beginning as violating international law (including the Charter of the United Nations). Legal experts note that Russia's invasion violates Article 2(4).
3. Why International Legal Experts and Institutions Have Rejected Russia's Legal Arguments
•No armed attack for self-defense: Experts have opined that the invasion violated Article 2(4)'s prohibition on the "use of force" and have generally rejected Russia's official legal justifications. One analysis states: "Under no scenario is Russia's armed invasion of Ukraine legal under contemporary international law and norms". It also states "Nothing that Ukraine has done to date could be construed as an 'armed attack' on Russia and justify any Russian claim to self-defense". Other analysis notes "There have been no threats of force against Russia from Ukraine nor from NATO member states... There is nothing to support a legal justification for Russia's military attack against Ukraine".
•Collective self-defense of Donetsk/Luhansk not recognized: Experts including John B. Bellinger III and others criticized the collective self-defense argument because these regions are not recognized as separate states under international law, and Ukraine has not threatened or attacked any other nation.
•Humanitarian intervention / genocide claim: Experts have rejected that the invasion is justified on humanitarian grounds to protect Russian-speakers, noting that the legality of humanitarian intervention is heavily disputed. One analysis notes intervening for humanitarian reasons "has not yet been widely accepted, unless the U.N. Security Council authorizes the intervention", and that even if Putin's false claim about targeting Russian-speakers were true, it would not justify the countrywide attacks. Russia's humanitarian justifications are widely perceived as a pretext and unsubstantiated by evidence of genocide.
•Comparisons to Western interventions: Russia has tried to justify its invasion by comparing it to interventions by the United States and allies in Kosovo, Iraq, Libya and Syria. These comparisons have been dismissed as irrelevant because "one illegal act does not make a different act legal" and "one illegal use of force does not justify another".
Because it violates the UN Charter and is more than a minor border incursion, Russia's military intervention has been qualified by legal experts as a crime of aggression under Article 8bis(1) of the Rome Statute.
The invasion also was said to violate other international agreements Russia is party to, including the 1975 Helsinki Final Act and the 1994 Budapest Memorandum, in which Russia agreed "to respect the independence and sovereignty and the existing borders of Ukraine".
4. Response at the United Nations
•Security Council: Russia vetoed the UN Security Council resolution that would have called for Russia to immediately cease its attack on Ukraine. Because Russia has a permanent seat and veto power, Security Council authorization for enforcement is unrealistic.
•General Assembly: The General Assembly overwhelmingly adopted a resolution demanding the Russian Federation immediately end its invasion of Ukraine and unconditionally withdraw all its military forces. On 28 February 2022, a resolution condemning the invasion was passed with 141-5 votes, with 35 nations abstaining. Later, the Assembly with 143 votes in favour condemned Russia's attempted illegal annexation of Donetsk, Kherson, Luhansk and Zaporizhzhia and demanded withdrawal, noting the regions are "temporarily occupied by Russia as a result of aggression" violating Ukraine's territorial integrity. The UN Secretary-General stated the use of force is "against the Charter" and "unacceptable".
Summary
From Russia's perspective, the invasion was presented as a necessary protective operation for Russian security and Russian-speaking populations in Donbas and to demilitarize/denazify Ukraine.
From the perspective of international law as interpreted by the UN Secretariat, the UN General Assembly majority, the International Court of Justice, and most international law scholars, the invasion lacks legal justification under the UN Charter because there was no armed attack on Russia to trigger self-defense, no Security Council authorization, and no widely accepted humanitarian intervention exception that would authorize a full-scale invasion and annexation of territory.