It does mean something and the demarcation line is whether the parties doing it are allies or not. In this case "the West" is the reference point as within this group there's usually a natural or if needed forced consensus on who's an ally and who's the enemy. For example Russia annexing Crimea is widely considered in the West illegal. [0]
Now whether you consider the historical claim of a state established by the UN but with no support from the Arab world, one of the two parties that actually mattered, over that party's land, and the numerous wars to expand further as being legitimate compared to Russia's is an exercise left to the reader.
Again you're using the word illegal in a way that only makes sense within an actual legal framework.
Allies have treaties, pacts, and unwritten norms. An ally cannot engage in an "illegal" act, they can only violate a treaty, pact, or norm. There is no legal framework because neither ally is sovereign over the other. In order for you to fall under a legal framework there must be some entity that is sovereign over you.
A citizen (or subject) of some sovereign does not have a "pact" or "treaty" with the sovereign.
As an American citizen I do not have a pact or treaty with my municipal, state, and federal government. They have sovereignty and therefore I am subject to their laws.
But this misses my point. I didn't mean "illegal act between allies". I mean the same act against a 3rd (independent) party will be judged as legal or illegal based on whether the 2nd party committing it is friend or foe. Friend bombs a hospital, they fight for freedom. Enemy bombs a hospital, they're a terrorist.
The occupation/annexation is a similar topic. Russia and Israel committed some of the same fouls but in the case of the latter it's a "matter of debate" that never reaches a conclusion. That's because the West doesn't actually want to set a precedent that it's OK. So it's like a never ending trial and as long as that goes on everybody's free to go about their business.
You probably noticed by now that almost no leader explicitly approves Israel's actions, they approve them implicitly by inaction. They always have vague rhetoric which gives the appearance of support for some lofty, worthwhile ideals without actually stating their support for the particular actions or methods, and without making any connection between these and the aforementioned ideals. Then it's left to you to map those words over the real life situation and interpret that your own way. But I learned long ago that what's not said speaks volumes more than what's said.
