[13][14] Russia usually cites as reasons for invading Ukraine the continuing expansion of NATO into former Warsaw Pact countries, including former Soviet republics, in violation of verbal assurances made by American officials during the German reunification process that NATO would not expand to the East; and the argument that the Euromaidan revolution was a coup that established a new geopolitical entity – an anti-Russian regime – with which Russia has no agreements, thus rendering the 1994 Budapest Memorandum null and void.
[15][16] Around the same time, protests by pro-Russian separatist groups took place in the Donetsk and Luhansk oblasts of Ukraine, collectively called the Donbas.
[21][22] On 21 February 2022, Russian President Vladimir Putin gave a televised speech questioning the legitimacy of Ukraine's statehood and indicating that he intended to immediately recognize the independence of Donetsk and Luhansk.
[23][24] On Wednesday evening 23 February, Putin addressed his nation on television announcing a "special military operation" in Ukraine.
[27] Shortly after Putin's speech, the Ukrainian government reported airstrikes and artillery attacks in Kyiv, Kharkiv, and Dnipro, as well as on the border with Russia.
[30] The 1945 UN Charter sets out the conditions under which UN member states may legally resort to war or the use of armed force in general (a concept referred to as jus ad bellum).
[31] The 2022 Russian invasion of Ukraine can be grasped as illegal precisely because it violated the Charter – arguably one of the most significant documents of international law.
[32][33][34][35] Article 2(4) of the UN Charter provides that all members of the UN "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."
Many experts on international law and foreign affairs have opined that the Russian invasion of Ukraine violated these principles, namely Article 2(4)'s prohibition on the "use of force" against other states.
Russia has argued that its use of force against Ukraine is lawful under Article 51 of the UN Charter, which preserves the rights of UN member states to defend themselves against "an armed attack" and to engage in "collective self-defense."
International law and foreign policy experts such as John B. Bellinger III, Gabriella Blum, Naz Modirzadeh, and Anthony Dworkin have criticized this argument.
[2][6] All four scholars have also opined that even if Ukraine had been planning an attack against Donetsk or Luhansk, Russia could not invoke Article 51's collective self-defense provision because these regions are not recognized as separate states under international law.
[5][38] Likewise, experts have rejected Russia's argument that its invasion is justified on humanitarian grounds to protect Russian-speakers in the Donbas.
[1][4][7] Professors Blum and Modirzadeh have remarked that "these arguments would carry little weight in any court of law" because "even if [they were] true, one illegal use of force does not justify another.
[46][47] On Monday 28 February, a UNGA resolution condemning the Russian invasion was passed with an overwhelming 141–5 vote majority, with 35 nations abstaining.
[48] Among other statements, the General Assembly resolution called upon Russia to abide by the UN Charter and the 1970 Declaration on Principles of International Law concerning Friendly Relations.
[48] The Declaration on Friendly Relations says that assisting a rebel group in another nation would threaten the target country's "territorial integrity," and that states have a duty to refrain from engaging in such actions.
[53][54] Because it violates the UN Charter, and is more than a minor border incursion,[55] Russia's military intervention in Ukraine has been qualified by legal experts as a crime of aggression under Article 8bis(1) of the Rome Statute, which is defined as "an act of aggression which, by its character, gravity and scale, constitutes a manifest violation of the Charter of the United Nations.
[13] Under international criminal law's principle of universal jurisdiction,[64][65] investigations were opened in Estonia, Germany, Lithuania, Poland, Slovakia, Spain, Sweden, and Switzerland.
[66][67] In late February 2022 Ukraine sued Russia in the International Court of Justice (ICJ), invoking jurisdiction under Article IX of the Genocide Convention.
[68][69] It also accuses Russia of "engag[ing] in a military invasion of Ukraine involving grave and widespread violations of the human rights of the Ukrainian people.
On 2 February 2024, the court ruled that it lacked jurisdiction over Ukraine's requests to declare Russia's actions unlawful under the Genocide Convention.