What constitutes a war crime?

Context

The International Criminal Court has announced that it will open an investigation into possible war crimes carried out in Ukraine.

About

About War Crimes:

According to UN Office of the High Commissioner for Human Rights, the term “war crimes” refers to serious breaches of international humanitarian law committed against civilians or enemy combatants during an international or domestic armed conflict, for which the perpetrators may be held criminally liable on an individual basis.
Such crimes are derived primarily from the Geneva Conventions of 1949 and their Additional Protocols I and II of 1977, and the Hague Conventions of 1899 and 1907.
Their most recent codification can be found in article 8 of the 1998 Rome Statute for the International Criminal Court (ICC).
In the case of an international armed conflict, any of the following acts committed against persons or property protected under the provisions of the relevant Geneva Convention:
- wilful killing;
- torture or inhuman treatment, including biological experiments;
- wilfully causing great suffering or serious injury to body or health;
- extensive destruction or appropriation of property, not justified by military necessity and carried out unlawfully and wantonly;
- compelling a prisoner of war or other protected person to serve in the forces of a hostile Power;
- wilfully depriving a prisoner of war or other protected person of the rights of a fair and regular trial;
- unlawful deportation or transfer;
- unlawful confinement;
- taking of hostages.

About International Criminal Court:

The International Criminal Court is an intergovernmental organization and international tribunal that sits in The Hague, Netherlands.
The ICC has jurisdiction to prosecute individuals for the international crimes of genocide, crimes against humanity, war crimes, and crimes of aggression.
Its founding treaty, the Rome Statute, entered into force on July 1, 2002.
It is intended to complement existing national judicial systems and it may therefore exercise its jurisdiction only when certain conditions are met, such as when national courts are unwilling or unable to prosecute criminals or when the United Nations Security Council or individual states refer situations to the Court.

War Crime vs Genocide:

The UN Office on Genocide Prevention and the Responsibility to Protect separates war crimes from genocide and crimes against humanity.
War crimes are defined as occurring in a domestic conflict or a war between two states, while genocide and crimes against humanity can happen in peacetime or during the unilateral aggression of a military toward a group of unarmed people.

Principles of war:

To decide whether an individual or a military has committed a war crime, international humanitarian law lays down three principles:
distinction
proportionality
precaution
Proportionality prohibits armies from responding to an attack with excessive violence. "If a soldier is killed, for example, you cannot bomb an entire city in retaliation”.
Precaution requires parties to a conflict to avoid or minimize the harm done to the civilian population.
The principle of distinction says that you have to be constantly trying to distinguish between civilian and belligerent populations and objects.