This is a question that has been extensively analyzed by international bodies, legal scholars, and governments, and there is a strong consensus view grounded in international law:
Under international law, the invasion is not justified.
Key points:
1.UN Charter violation: The UN Charter prohibits the use of force against the territorial integrity or political independence of any state (Article 2(4)), with narrow exceptions for self-defense or UN Security Council authorization. Russia's invasion of a sovereign, internationally recognized state does not fit either exception.
2.UN General Assembly response: The UN General Assembly passed resolutions (with overwhelming majorities, though not unanimous) condemning the invasion as a violation of Ukraine's sovereignty and territorial integrity, and demanding Russian withdrawal.
3.International Court of Justice: The ICJ ordered Russia to suspend military operations, finding that Russia's justification (alleged genocide in Donbas) was not adequately substantiated to invoke the legal doctrine Russia claimed.
4.Russia's stated justifications — including claims of protecting Russian-speakers from "genocide," preventing NATO expansion, and "denazifying" Ukraine — have been widely rejected by independent legal analysts, historians, and most of the international community as not meeting the legal thresholds for the use of force, and in some cases (like the genocide claim) as factually unsubstantiated.
5.Self-determination arguments: Some sympathetic to Russia's position point to NATO expansion concerns or the situation of Russian speakers in eastern Ukraine as grievances. These may explain Russia's motivations, but grievances or security concerns generally do not, under international law, justify a full-scale invasion of another sovereign nation.
Where reasonable disagreement exists: There are debates about the wisdom of Western/NATO policy choices leading up to 2022 (e.g., NATO enlargement, the Minsk agreements' implementation), and different geopolitical schools of thought (realist vs. liberal internationalist) weigh these historical factors differently. But this is distinct from the legal and moral question of whether invading and attempting to annex parts of another country is justified — on that narrower question, the international legal consensus is clear: it is not.
If you're interested, I can go deeper into any specific dimension — the legal arguments, the historical lead-up, competing geopolitical theories, or how public opinion varies globally on this question.