What does "legal" even really mean between states at war. The consequences typically come down to a popularity contest and Iran is one of the few states with fewer friends than Israel.

Was Iran's activities funding militias in Lebanon, Syria, Iraq and Yemen, which launched attacks against Israel and US forces "legal"? Which of the US' activities were "legal"? It's all mostly a bad joke.

Countries can attack others. There is not like a superset of a country over all countries that says what is and isn’t legal. All we have are agreements and treaties.

Not that we would or should but the US could attack any number of countries today and only if one or more countries stopped the US would the victor be able to say it’s illegal.

This is between nation states. Concepts like laws and legality really don’t apply at this level of abstraction. Agreements are a matter of convenience and convention because there is no higher authority that can enforce them.

Geopolitics operates in an explicitly anarchic arena.