Article 2(4) of the UN Charter reads: ” 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 Purposes of the United Nations.” […]
Russian behaviour fulfils all the elements of the violation. Given the timing, the geographical location, the exceptional nature of the manoeuvres in which an intervention is being played out, one may credibly infer that Russia is declaring its willingness to use force to achieve concrete political goals […]
If Russia intervenes militarily, Ukraine would be entitled to defend itself. It could also request assistance from other States. Military action in exercise of the collective right of self-defence is lawful provided it meets further conditions. In particular, the response would have to be proportionate.
As a side note, a mere border skirmish would still not entitle Ukraine (and allies) to self-defence. Although Russia would already violate the ban on the use of force with minor incursions, there would not yet be an “armed attack” in the technical sense that would trigger the right to self-defence under the UN Charter. This gap between the prohibition of violence and the right of self-defence means that low-level violence must be accepted by the victim State.
However, it is difficult to assess the preceding threat solely on the basis of a backward projection of the legitimacy of a (hypothetical) military response. It remains speculative how Western military powers would react. They would only be entitled to (collective) self-defence if a Russian armed attack were imminent or already underway. It would not be permissible to take “pre-emptive” action, for example, on the basis of intelligence information without objective indications. Such an anticipation of the right of self-defence has been claimed by the USA, especially in the wake of the 9/11 attacks, but was not accepted by the community of states. However, there is a grey area of lawful self-defence against imminent armed attacks. Läs artikel