Once upon a Time in Nuremberg...
It was the 20th of November 1945. 24 people were brought to the bench and prosecuted for many charges that stemmed from their conduct in WW2. Among those charges was the conglomerated concept of Crimes against Humanity, brought against 20 of the accused. The charging documents defined this as:
Murder, extermination, enslavement, deportation, and other inhumane acts committed against any civilian population, before or during the war, or persecutions on political, racial or religious grounds in execution of or in connection with any crime within the jurisdiction of the Tribunal, whether or not in violation of the domestic law of the country where perpetrated.
While the charge title was new, it was just a bundle of things already illegal in warfare under the laws and customs of war.
Of these 20 charges for Crimes against Humanity, 16 ended in convictions, one was retracted for medical reasons, 1 accused committed suicide, and 2 were found not guilty. Of the convicted, Bormann was tried in absentia, because he could not be found. As it appeared later, he had been dead for half a year when the charges were brought.
The partial aspects of the Crimes against Humanity charge all require premeditation and intent: Murder is a killing with intent, extermination is a form of mass murder, enslavement can only be done with intent to deprive someone of their rights, deportation differentiates from evacuation in the intent and reasons why it is done, and inhuman acts are specified insomuch that they must be bound to other crimes already alleged.
From Nuremberg to Rome
Since the Nuremberg Trials, the Geneva Conventions put explicit protection on the civilians and made Genocide not just illegal from other charges but as a chargeable point itself. Such charges were brought in the Tribunal for Yugoslavia, where the charge for crimes against humanity borrows heavily from the Nuremberg definition:
The International Tribunal shall have the power to prosecute persons responsible for the following crimes when committed in armed conflict, whether international or internal in character, and directed against any civilian population:
(a) murder;
(b) extermination;
(c) enslavement;
(d) deportation;
(e) imprisonment;
(f) torture;
(g) rape;
(h) persecutions on political, racial and religious grounds;
(i) other inhumane acts.
All those charges but (i) require intent in themselves, but those are not used to prosecute genocide anymore. They do appear next to a genocide charge very often, as a complementary charge of crimes against humanity. For example Milošević was charged with this, besides genocide and complicity in genocide. The Genocide charge in turn stemmed from UN Resolution 96(1):
Genocide is a denial of the right of existence of entire human groups, as homicide is the denial of the right to live of individual human beings; such denial of the right of existence shocks the conscience of mankind, results in great losses to humanity in the form of cultural and other contributions represented by these human groups, and is contrary to moral law and the spirit and aims of the United Nations. Many instances of such crimes of genocide have occurred when racial, religious, political and other groups have been destroyed, entirely or in part.
This was then refined into the Genocide Convention Article 2:
any of the following acts committed with intent to destroy, in whole or in part, a national, ethnic, racial or religious group, as such:
(a) Killing members of the group;
(b) Causing serious bodily or mental harm to members of the group;
(c) Deliberately inflicting on the group conditions of life calculated to bring about its physical destruction in whole or in part;
(d) Imposing measures intended to prevent births within the group;
(e) Forcibly transferring children of the group to another group.
The Genocide convention calls out the requirement for intent.
Rome Statute
The Rome Statute Article 6 is the prime charge against genocide since its passing in 1998, and it is still right next to the very related Crimes against Humanity in Article 7:
Article 6 - Genocide
For the purpose of this Statute, "genocide" means any of the following acts committed with intent to
destroy, in whole or in part, a national, ethnical, racial or religious group, as such:
(a) Killing members of the group;
(b) Causing serious bodily or mental harm to members of the group;
(c) Deliberately inflicting on the group conditions of life calculated to bring about its physical destruction
in whole or in part;
(d) Imposing measures intended to prevent births within the group;
(e) Forcibly transferring children of the group to another group.
The intent is outright called out. However, there are still Crimes against Humanity that can be leveraged against people that can't be proven with the intent of genocide, insofar as the actions that lead to the deaths are under that charge.
