The rules of war

These rules apply to both Gaza and Ukraine. PD kindly provided the links to these info sources.

The ICRC has a FAQ page directed to Qs&As about the rules of war:

Even wars have rules. What does that mean?

It means: You do not attack civilians. You limit as much as you can the impact of your warfare on women and children, as well as on other civilians. You treat detainees humanely. You do not torture people.

Here's one Q&A:

Do civilians picking up arms lose their protection against direct attack under IHL (international humanitarian law)?

It depends. IHL defines civilians as anyone who is neither a member of State armed forces, nor a member of an organized armed group with a continuous combat function, nor a participant in a levée en masse.

Civilians are protected against direct attack unless, and for such time, as they directly participate in hostilities. Parties to an armed conflict must take all feasible precautions in determining whether a person is a civilian and, if that is the case, whether he or she is directly participating in hostilities.

In case of doubt, the person in question must be presumed to be a civilian and protected against direct attack. To protect civilians, combatants – and anyone directly participating in hostilities – must distinguish themselves from civilians in all military operations by wearing identifiable insignia and carrying arms openly. The ICRC has issued Interpretive Guidance which provides recommendations concerning the interpretation of IHL as it relates to the concept of direct participation in hostilities.

IHL related to modern warfare sieges (pages 1-2 of 16):