It's tricky. Arming a country or group than then launches an attack, or uses those weapons in a war, doesn't make you a participant in that conflict. This is why Europe and the US can supply weapons to Ukraine without being participants in a conflict with Russia.

However Iran has the stated intention of destroying the state of Israel, and actively incites it's proxies to attack Israel, and this could be seen as a valid justification for taking action. Not a lawyer though.