This is a question about a matter of significant international consensus and legal fact, not a genuinely contested issue where reasonable people disagree based on differing values. Let me give you a clear, factual answer.
No, Russia's invasion of Ukraine is not justified under international law. This is the overwhelming consensus of international legal bodies, most of the world's governments, and international institutions.
Key facts supporting this:
1.UN Charter violation: The invasion violates Article 2(4) of the UN Charter, which prohibits the use of force against the territorial integrity or political independence of any state. Russia's actions constitute a clear act of aggression.
2.UN General Assembly condemnation: The UN General Assembly has repeatedly passed resolutions (with large majorities, often 140+ countries) condemning the invasion and demanding Russian withdrawal.
3.International Court of Justice: The ICJ ordered Russia to suspend military operations in Ukraine as a provisional measure in 2022.
4.International Criminal Court: The ICC issued an arrest warrant for Vladimir Putin, alleging war crimes.
5.Rejection of Russia's stated justifications: Russia has offered various justifications (claims of NATO threat, protecting Russian-speakers, "denazification" of Ukraine). These have been widely rejected by international legal scholars and the international community as pretexts that do not meet the narrow legal exceptions for use of force under international law (which generally require self-defense against an actual armed attack or UN Security Council authorization).
While there are legitimate academic debates about the geopolitical causes leading up to the war (e.g., discussions about NATO expansion, security dilemmas), this is distinct from the legal and moral question of whether the invasion itself—a large-scale armed attack on a sovereign neighboring state—is "justified." On that specific question, there is broad international consensus that it is not.
If you're interested, I'm happy to discuss the historical/geopolitical context that led to the conflict, which is a more nuanced area with legitimate scholarly debate.