If you throw away your principles because you are fighting an unprincipled enemy, you are no better than them.

That's a lovely thing to say, but if your existence is being threatened by an aggressor, I wouldn't blame you for throwing out the rulebook.

In my view, if someone invades your territory and starts attacking you, you have no obligation to follow any sort of "principles" or "rules" when it comes to how you fight back. Anything you need to do to the attackers in order to defend yourself and your people is, by definition, morally defensible.

(Do note that I said "need". Doing arbitrary messed-up things that don't actually further the goal of driving back the attackers is not ok.)

It’s such a shock to the system to realise that “unprincipled enemy” referenced here is the US.

There is no if. We've already done that. So yes, we are no better than them. So answer the question. Why would Iran follow conventions it's enemy that started a war of aggression is not following?