Does combatants include Wagner mercenaries?, If so, update the OP?

@Jen indeed,Ukraine does see itself at war by fact of invasion, even if there was no declaration. Also note, that the Russian embassy was evacuated the day before hostilities began. All diplomatic ties to russia have been severed from Ukraine the same day.

It depends on who is doing the deciding: some may say terrorists, some may say soldiers, some may say a combination of both. It's possible that sometime in the future a court (national or international) will make a determination based on all the available facts and according to their definition of terrorism.

@Trish Questions of international law and law of war are still questions of law and are on-topic here.

@Trish whether the third Geneva convention applies depends on whether the facts presented meet the requirements of its second article. Sufficient facts to make this determination are public knowledge. Would it be off topic to ask why a particular murder was or was not prosecuted as a hate crime or an act of terrorism?

@phoog good point. If you read the question as "Does this article apply to the situation" it is on topic. I had read it as "Why does Ukraine believe it is at war without one declared?", which would be politics.

@DavidSiegel reading it as phoog suggested, it certainly is. Retracted.

Aside from international ‘law’ which is always more vague than state law, there is also little to gain by Ukraine by not treating their PoW’s well. Russian soldiers don’t seem overly motivated in this war, but could become so by horror stories about prisoner treatment. Right now it’s fairly easy to just surrender.

@SebastiaanvandenBroek You seem to incorrectly assume that there are no such horror stories (made up or otherwise) about POW treatment in Russian media. IMO the real reason is to uphold an image for the US / Europe, which may refuse to provide help to a country which commits atrocities against POWs.

According to Russia, those troops aren't in Ukraine. They are in the newly annexed regions that are now part of Russia. Of course, nobody outside of Russia agrees with that, but that's their story and they're sticking to it.

@DmitryGrigoryev: Another consideration is that the prisoners will eventually be going back to Russia, likely at a time when its government will be somewhat malleable. Prisoners who are treated well may, after returning to Russia, help ensure that a new government recognizes Ukraine's right to exist.

Standing army in uniform.

Because it's the right thing to do?

@Mazura I explicitly asked for a legal reason.

@SebastiaanvandenBroek Does Ukraine treat their criminals badly? Surrendering may be seen as an act of active repentance.

@AlanB If a criminal organization dresses its "soldiers" in uniforms, does law of war apply to them? If not (not a state-actor and not a High Contracting Party of the III Geneva Convention), that makes interesting case and I may need to ask a second question about soldiers of DPR/DNR and LPR/LNR.