I'll gather the legal and historical context to assess the invasion's justification.
Russia's 2022 invasion of Ukraine is described differently depending on the framework used — Russian stated justifications vs. international law and the position of most states and international courts.
1. Russia's stated justifications
In his February 24, 2022 address, President Vladimir Putin named the official reasons for the "special military operation" as:
•“the protection of the Russian-speaking population in Ukraine from genocide, the de-nationalization and demilitarization of Ukraine, as well as the prevention of the growing threat to Russia's national interests - the expansion of NATO to the east"
Other descriptions of Kremlin justifications summarized them 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
•Following a request from the authorities of the Donetsk and Lugansk republics for assistance, and to protect the people exposed to humiliation and genocide, seeking demilitarization and denazification
A related argument cited by Russia is opposition to NATO's expansion into Eastern Europe, viewing it as a threat to its security and claiming it was acting preemptively to protect its security.
In both instances Russia invoked international law and the UN Charter to justify its invasion.
2. International law framework
The central rule is in the U.N. Charter:
> “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 permits only two exceptions to this prohibition:
•self-defence only in response to an armed attack under Article 51, and collective action authorized through the UN Security Council to restore peace and security. Alternatively, the Security Council can authorize military action to maintain international peace.
Legal analyses cited in search results conclude:
•"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 to prohibiting the use of force is self-defense, “if an armed attack occurs”
•"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 countrywide attacks
•Russia's justifications including claims about protecting Russian speakers or “denazifying” Ukraine, fit neither exception
•Russia's invasion violates Article 2(4) of the U.N. Charter
Intervening for humanitarian reasons has been asserted as another rationale, but this justification has not yet been widely accepted, unless the U.N. Security Council authorizes the intervention. Countries cannot avoid obligations by pretending actions are peacekeeping missions, as Putin has said.
3. Positions of UN bodies and courts
•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
•Member States overwhelmingly adopted a resolution demanding the Russian Federation immediately end its invasion of Ukraine and unconditionally withdraw all its military forces
•The International Court of Justice (ICJ) ruled by a vote of 13 to two that Russia shall immediately suspend military operations, in response to Ukraine's suit accusing Russia of manipulating the concept of genocide to justify its military aggression. Although the ICJ's verdicts are binding, there are doubts whether Moscow will abide and the court has no direct means of enforcing its rulings.
Summary
•Under prevailing international law as interpreted by the UN Secretary-General, the UN General Assembly, the ICJ, and most international law scholars cited above, the invasion is not considered legally justified because neither self-defense against an armed attack nor Security Council authorization applied.
•Under Russia's own stated framework, the invasion is presented as justified as protection of populations in Donetsk and Luhansk, denazification/demilitarization, and prevention of NATO expansion as a security threat.
Whether the invasion is viewed as "justified" therefore depends on whether one applies the Russian political/security rationale or the UN Charter-based legal standard accepted by the large majority of states.