The Hague Conventions of 1899 and 1907, a set of multilateral treaties, established laws and rules that warring parties must adhere to.
The definition of a war crime against civilians was enshrined in Article 8 of the Rome Statute of the International Criminal Court and is based on the Geneva Conventions of 1949.
While politicians and observers say that war crimes are being committed in Ukraine, until "the evidence is fully substantiated and double-checked and confirmed, for the time being we can talk only about alleged war crimes," says Maria Varaki, a lecturer in international law and co-director of the War Crimes Research Group at King's College London.
Lawyers refer to war crimes as grave breaches of the Geneva Conventions. "We're talking about willful killing of civilians, about torture, about forced displacement, about indiscriminate attacks. Because one of the fundamental principles of the laws of war is that civilians should not be targeted," said Varaki.
In that sense, the attacks on schools and maternity wards in Kyiv or a theater in Mariupol qualify as violations of those laws.
"And in the last 48 hours, we've seen these people with civilian clothes on the streets [in Bucha] and some of them had been executed with one bullet to the back of their head. So, these are atrocities under international humanitarian law," said Varaki.
Legal gray areas
As with most aspects of war, there are grey areas within the definition of what constitutes a war crime. To that end, international humanitarian law is guided by three principles — distinction, proportionality and precaution — that stipulate that parties to a conflict cannot target civilians or civilian objects.
While on paper these principles appear to be unequivocally clear, they are open to interpretation, for example, if a civilian object is deemed to be a military objective based on its use, purpose and function.
"I can give you an example of a shopping mall that was bombarded. The Ukrainians said this was clearly civilian infrastructure. And the Russians said they had intelligence that it was being used as a storage place for military purposes," said Varaki.
While there are rules to limit human suffering, those principles are often abused and manipulated.
"Everything is based on interpretation and on human judgment: when you should target, what you should target, and to what extent," said Varaki.
Rape and sexual assault
Rape and sexual assault are considered war crimes and a breach of international humanitarian law. This was enshrined in Article 27 of the 1949 Geneva Convention. The ad hoc tribunals and the Rome Statute of the ICC defined a detailed list of gender-related crimes.
A 2008 UN resolution (1820) reaffirmed that rape can be a war crime and a weapon of war.
Both Ukraine's prosecutor general and the International Criminal Court have said they will open investigations into reported sexual violence.
However, therein lies the rub, said Varaki.
"The issue there is how easily that would be documented. We are talking about extremely vulnerable victims. My understanding is that the Russians tried to cover up some of those crimes by burning victims of sexual violence."
Notwithstanding those attempts to conceal the crimes, she expects that more and more incidents will come to light and be investigated. "We have victims who came out of the Russian-controlled territories who are now sharing their horrific stories with the world."
Genocide
Ukraine's president, Volodymyr Zelenskyy, has accused Russian forces of carrying out genocide. It refers to the deliberate attempt to destroy a national, ethnic, racial or religious group.
Again, however, we're in murky legal waters. Genocide, technically a crime against humanity, can also happen during peacetime, not just in a war or conflict situation.
"Legally speaking, it's very difficult to prove the crime of genocide. You need genocidal intent in order to prove that they want to exterminate a particular group of people according to the crime of genocide we have in the Rome Statute, but also in the 1948 Genocide Convention," said Varaki.
She points out that it's important to assess the word genocide in terms of its semiotic importance. "It has this particular emotional value. When politicians invoke that, they trigger a particular emotional reaction."
"So, politicians use the word genocide for other purposes. But legally speaking, it's very, very difficult to prove the crime of genocide."