. . . under any sort of international law?"
Accountability is out of fashion here. People call Trump "President Trump", but he is an insurrectionist. He belongs in prison. The courts here are so broken that even the simple act of convicting him for his obvious crimes was not possible as things stood and as they still currently stand.
Furthermore, that is not even the first case of a serious lack of accountability. Who wrote the torture memos that granted the Bush Administration the legal 'cover' to practice forms of torture? Were those individuals prosecuted or promoted? Where is the US Senate's full report on torture during W's Administration? Has anyone again considered prosecuting W for those war crimes which were committed under his leadership?
There is no systemic form of accountability available. In lieu of that more formal and judicial accountability and because we seem generally to ignore possible consequences (some to be expected, some completely disconnected from our common worldviews), it seems likely that the US has another large-scale terrorist event awaiting us in the future. Bin Laden's complaints that motivated 9/11 were less accessible to our perspectives, but striking a girls' school and killing over 160 would seem an undeniably motivating factor to someone who would want to seek revenge. Even if the Iranian military is incapable of finding a way to seek revenge, the fact that Trump chose this conflict, places all the blame on us, and that offers a clear, diffuse target to anyone or anyone's group who would either want revenge or even just want a cause about which to coalesce.
Aside from the serious issues related to the morality of all of that, Trump's severe incompetence as President has degraded the ability of the DHS to protect the US and people in the US from attacks. So, not only has Trump given others a cause to attack the US, but he simultaneously has weakened our ability to defend against such an attack.
Given the last twenty years of US political history, does anyone expect this delusional lack of accountability and its ramifications to change any time soon? I don't.