I'll examine the legal arguments around Russia's invasion and how the international community has assessed them.
Russia's full-scale invasion of Ukraine on 24 February 2022 has been justified differently by Russia and by most of the international community:
1. Russia's Stated Justifications
President Vladimir Putin announced a "special military operation" claiming it was necessary to:
•"protect people who have been subjected to abuse and genocide" by the Ukrainian government, stating on Feb 24 2022 the purpose was "to defend people who, for 8 years, have faced humiliation and genocide from the Kiev regime"
•fulfill requests for assistance from the Donetsk and Luhansk People's Republics — which Russia recognized as independent states two days before the invasion
•achieve "the demilitarisation and denazification of Ukraine". In public speeches and state media, Putin framed the operation as protection for residents of the separatist Donetsk and Luhansk regions, a mission to "demilitarise and denazify" Ukraine, and a response to alleged threats posed by NATO expansion
•protect pro-Russian/Russian-speaking factions, especially in Crimea and the Donbas, and to challenge NATO expansion along Russia's border
Legally, Russia argued its use of force was lawful under Article 51 of the UN Charter, which preserves the rights of UN members to defend themselves against "an armed attack" and to engage in "collective self-defense", specifically to defend Donetsk and Luhansk after recognizing them. It also invoked humanitarian grounds and prevention of alleged genocide.
Scholars described Putin's allegations of genocide and comparison of Ukraine to a Nazi state as baseless.
2. Assessment Under International Law
The dominant view in international law and at the United Nations is that the invasion violates the UN Charter:
•The 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". The wording is "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 UN Charter are self-defense and an authorization by the Security Council. Article 2(4) 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, or in any other manner inconsistent with the Purposes of the United Nations".
•Analysis: "Under no scenario is Russia's armed invasion of Ukraine legal under contemporary international law and norms". "Self-defense is the only justification for use of force against another country, according to international law. This condition is found in the U.N. Charter and is binding for all 193 U.N. member countries". The Charter's only clear exception is self-defense, "if an armed attack occurs".
•Experts concluded: "Nothing that Ukraine has done to date could be construed as an 'armed attack' on Russia and justify any Russian claim to self-defense".
•Even if claims about targeting Russian-speakers were true, "it would not justify the countrywide attacks", and humanitarian intervention without Security Council authorization "has not yet been widely accepted".
Specific rejections of Russia's legal arguments:
•Experts such as John B. Bellinger III and others criticized the self-defense argument because Ukraine has not threatened or attacked any other nation, and because Donetsk and Luhansk "are not recognized as separate states under international law".
•Experts have also rejected the humanitarian/genocide justification as "widely perceived as a pretext, and are unsubstantiated by any evidence that Ukraine has committed, or is committing, any acts against Russians in Donetsk and Luhansk that could amount to genocide", and question whether international law even allows force to remedy genocide absent Security Council authorization.
•Comparisons Russia has made to Western interventions in Kosovo, Iraq, Libya and Syria have been dismissed as "one illegal use of force does not justify another".
3. United Nations and Courts Response
•The UN Secretary-General said Russia's invasion "directly conflicts with the United Nations Charter", stating "The use of force by one country against another is the repudiation of the principles that every country committed to uphold... It is wrong. It is against the (United Nations) Charter".
•The UN General Assembly, in its resolution of 2 March 2022, "immediately qualified Russia's war against Ukraine as an act of aggression in violation of Article 2(4) of the UN Charter".
•The General Assembly "overwhelmingly adopted a resolution demanding the Russian Federation immediately end its invasion of Ukraine and unconditionally withdraw all its military forces". The vote on 28 February was 141-5 with 35 abstentions.
•The EU reiterates its condemnation of "Russia's illegal, unprovoked and unjustified war of aggression, which constitutes a manifest violation of international law".
•At the Security Council, Russia vetoed a resolution that would have called for it to immediately cease its attack, preventing Security Council enforcement because Russia holds veto power as a permanent member.
•The International Court of Justice (ICJ) on 16 March 2022 ordered Russia to "immediately suspend the military operations" in Ukraine, saying it had not seen any evidence to support the claim that Ukraine was committing genocide against Russian-speakers. The UN's report described the invasion as violating the Charter and as "illegal precisely because it violated the Charter".
•Legal experts have qualified the invasion as violating international law including the Charter and as a "crime of aggression" defined as "an act of aggression which, by its character, gravity and scale, constitutes a manifest violation of the Charter" including "invasion or attack by the armed forces of a State of the territory of another State".
•The invasion is also said to violate other 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".
In summary, under Russia's own stated framework the operation is presented as justified as collective self-defense, protection of Russian speakers, denazification, demilitarization, and countering NATO/genocide threats. Under the UN Charter and the prevailing interpretation of international law by the UN General Assembly, the ICJ, and most international law scholars, those justifications do not meet the legal criteria for use of force, and the invasion is therefore considered illegal and an act of aggression.