Dianavan has said it. It's that simple. That is the dividing line, and when you cross it and you invade another country that has not already attacked you in a similar fashion and on a similar scale (meaning all-out war), you have committed aggression. That's a very major illegality, in fact it's the biggest illegality possible, short of deliberate genocide.

Who has been guilty of that in the Middle East in recent times? Israel (invaded Lebanon), Iraq (invaded Iran), Iraq again (invaded Kuwait), the USA and the UK (invaded Afghanistan), the USA and the UK (invaded Iraq), and Israel (invaded Lebanon again).

That's 7 clear cases of aggression. All illegal. None committed by Iran. 7 of them committed by members of the USA/UK/Israel coalition or by an ally of the USA. (Saddam was an ally of the USA at the time he assaulted Iran and at the time he assaulted Kuwait. He ceased to be an ally of the USA immediately after his assault began on Kuwait.)

4 of those cases of aggression occured right next to Iran, right by its borders, and one of them occured on Iran's territory, killing a great many Iranians. Just who is the injured party supposed to be here? And who should feel a need to protect themselves strenuously?

Iran, that's who. They have suffered a good deal more from outside attack than Israel has, if you want to go by body count or any other yardstick you could care to mention.