Ukraine doesn't have to explain itself
Are you wondering why Russia is getting away with threatening to invade Ukraine? The answer is simple - there is no coherent and effective regulation of the use of force at the international level and of organising a response to it.
The United Nations Charter prohibits the use of force by States in Article 2(4). It 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, or in any other manner inconsistent with the Purpose of the United Nations.” Article 2 (3) imposes an obligation to settle disputes, including territorial and boundary disputes, peacefully. Self-defence is the main exception to the rule on the use of force prohibition.
Article 51 of the UN Charter provides: “Nothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the UN, until the Security Council has taken measures necessary to maintain international peace and security. Measures taken by Members in the exercise of this right of self-defence shall be immediately reported to the Security Council and shall not in any way affect the authority and responsibility of the Security Council under the present Charter to take at any time such action as it deems necessary in order to maintain or restore international peace and security.”
Self-defence must be necessary and proportionate. It is generally taken as limiting self-defence to action which is necessary to recover territory or repel an attack on a State’s forces and which is proportionate to this end. Armed attack extends beyond attacks by regular forces; it can also cover attacks by armed bands, irregulars, terrorists and mercenaries.
There are many debates regarding the scope of different terms in Article 51. For example, does the right to self-defence occur only after the attack has started or also when the State is anticipating an imminent attack? The USA has already used a wide interpretation on several occasions: in the Gulf during the Iraq/Iran war in 1980-1988, in the no-fly zones in Iraq during the Iraq/Kuwait War in 1990. Israel did that as well in 1981 in Iraq.
The International Court of Justice refused to clarify the definition of an act of aggression, armed attack and pre-emptive self-defence. This is very unfortunate because with the absence of legal definitions - international law is formed by the actions of States (customary law occurs, or at least a generally accepted notion of permitted behaviour).
Therefore, it is not a surprise that Russia helped itself to invading Ukraine in 2014 using the wide interpretation of Article 2 (4) and 51: it interpreted the political course change in Ukraine as a threat to its own state regime and also claimed to be protecting “Russian citizens” (which apparently includes everyone who knows Russian language…). All this is unconventional doctrinal interpretations of very serious norms that guard world peace. Some countries get away with it… Only many years later the International Court of Justice or the International Criminal Court might decide otherwise.
However, Ukraine doesn’t get a chance to use Article 2 (4) and 51 of the UN Charter in its conventional general interpretation - it definitely has the right to respond to what happened in 2014 - the annexation of Crimea and the effective control of Russia over Donbas. Even now, when Ukraine is under a very real threat of yet another use of force by Russia - the United Nations is silent. OSCE didn’t even hold an official meeting.
Nevertheless, even if international law is barely respected - Ukraine does have every right to stand its ground, protect its borders and doesn’t have to explain to Russia the arms supplies it received from its Western partners.