People like Hegseth like to think that the rules of warfare are woke gay bullshit designed by losers to hamper the natural winners (i.e. them), but they don't understand that prior to being codified by the Geneva convention, the rules of war were customary, and existed for reasons other than purely humanitarian ones.
For instance, when surrendering a city or fort, the defenders would be typically given something called honours of war, in which they would be permitted to march out bearing their arms and waving their flags, thus avoiding humiliation. They would then surrender their arms and be taken as prisoners of war, or, supposing that the attacking army doesn't have the facilities to keep POWs, would be granted free passage. Now you could simply let them march out and then massacre them, thus avoiding either having to care for POWs or having a bunch of enemy troops that you have to deal with in a future battle, but you can only pull that trick once - the next time, the defenders will fight to the death rather than be massacred. So you pull a dirty trick to win this battle, but it makes every future battle harder, and it also means that they will not be likely to take you prisoner. Even the Mongols would typically honour the surrender of a city (provided no resistance was offered after the ultimatum), because besieging or storming a city is much costlier than making it surrender.
You see this effect in action in the present war, where the US twice used diplomatic talks as cover for suprise attacks, and thus Iran no longer has any reason to see any diplomacy with the US as legitimate, so even as Trump wants to pull out, he can't.You can assassinate their civilian leaders, but now you have to keep your head on a swivel as you're also suddenly a legitimate target. You can sink their defenceless ship and leave the survivors to drown, but now they have no reciprocal obligation towards your crews. You can strike their embassies, but now your embassies are struck in return. You can launch attacks on them from nominally neutral countries, but now those countries are legitimate targets.
