Iranian backed militias under instructions from Quuds force sacking the US embassy is something that cannot stand. That was justification enough for almost anything. That is the one rule of international relations that doesn’t get broken. Except by Iran that has never respected it.
That’s all the justification the US needed. It’s that big a deal. It’s as simple as that. Whether it is wise is a whole other discussion. But the way this conversation has gone shows people have gotten very used to the US taking blows and doing nothing, and not reminding all in sundry how things work.