Reflecting on this, I think it’s fair to consider the International Court of Justice (part of the UN) to be a legal system with legitimate jurisdiction over most countries - even if it’s frequently unable to enforce its law. And therefore it’s reasonable to describe a war as “illegal”, wrt the UN.
But I do believe it’s a pointless description - I can’t think of any legal wars, especially if one believes committing war crimes makes even a UN-sanctioned war illegal. I consider it a propagandic description used to put spin on a war. (And just adding that on a personal level, I believe legality is irrelevant to morality and acceptability)
Retaliation is generally understood to be self defence, as a deterrent against further attacks.
While the statement may be true, I want to emphasize that a common tactic is for a country to harass or suppress another country until they retaliate, and claim that retaliation is in fact unprompted aggression which must be retaliated against. While there are notable cases of this in the past decade, this tactic is tried and true across centuries. Therefore, we often see wars where both sides claim self-defense, and both their blocs generally understand their side to be justified.









On the other hand, if my workplace was on a list of announced missile targets, I’d stay away.
Assuming that there’s no real chance of their enemy setting up a defense in time: