Livvy, he is wrong. It is not a violation of International Law.
It’s nasty, but it is no violation and no war crime (I also posted the video above, not that I mind that it’s posted more).
It would have been a violation had it not been in international waters (for instance in Indian waters). But since it is in International Waters, it is very legal. There is no advanced warning given to war-ships when at war before you shoot at them, unless you want to do that. But it is not required by law.
I posted the video because I wanted to make a point that it is ungentlemanly conduct and that it was unecessary to kill them (at least sink it without warning), but it is not a war crime.
The war crime is launching the war. But using a submarine to sink a frigate is not at all a war crime.
https://ihl-databases.icrc.org/en/customary-ihl/v1/rule20
Note here, it says merchant vessel, not military craft:
https://ihl-databases.icrc.org/en/ihl-treaties/submarine-warfare-rules-1930/article-22
War is hell, usually also very dishonourable. Ambushes are common.
The problem is the war itself, that is illegal. But when a war has started, that crime is set aside and it does not automatically become a crime if individual units shoot at other units. The crime is going to war, not fighting it (unless war crimes are involved in fighting it, such as targeting of civilians and etc.)

