Iranian naval exercises in the Gulf of Oman devolved into disaster Sunday afternoon when a missile from one Iranian ship accidentally struck another, killing 19 and injuring 15 more. The friendly-fire incident is the second large-scale, deadly error the Iranian military has made this year after it mistakenly shot down a passenger jet departing Tehran.

Now, as then, the situation is tragic but also instructive for U.S. foreign policy: “Firing at your own targets, whether military or civil, in such a short space of time, is not human error. It’s a catastrophic failure of management and command,” said Maziar Khosravi, an Iranian reformist-linked journalist. It’s indicative of how weak the regime in Tehran truly is — and how incapable it is of posing an existential threat to the United States. It’s a reminder that the only way Iran can reliably threaten American lives is if Washington keeps them within Iran’s limited reach in the Middle East.