I think to answer this, we should look at the history of the coining of the term genocide and additionally how the UN defines it, since the UN is the international body that tries accused criminals to determine if/when/how certain actions meet the legally defined terms of genocide, war crimes, and or crimes against humanity. I'll also be drawing from Genocide: A Comprehensive Introduction by Adam Jones, Blood and Soil: A World History of Genocide and Extermination from Sparta to Darfur by Ben Kiernan and Axis Rule in Occupied Europe by Ralph Lemkin.

Ralph Lemkin was the scholar who coined the term genocide and his writings were used to help define and write the Genocide Convention in 1948 when the UN adopted the Convention on the Prevention and Punishment of the Crime of Genocide. Lemkin wrote in Axis Rule that

"By 'genocide' we mean the destruction of a nation or an ethnic group.... Generally speaking, genocide does not necessarily mean the immediate destruction of a nation, except when accomplished by mass killings of all members of a nation. It is intended rather to signify a coordinated plan of different actions aiming at the destruction of essential foundations of the life of national groups, with the aim of annihilating the groups themselves. The objectives of such a plan would be disintegration of the political and social institutions of culture, language, national feelings, religion, and the economic existence of national groups, and the destruction of the personal security, liberty, health, dignity, and even the lives of the individuals belonging to such groups. Genocide is directed against the national group as an entity, and the actions involved are directed against individuals, not in their individual capacity, but as members of the national group. ...Genocide has two phases: one, destruction of the national pattern of the oppressed group; the other the imposition of the national pattern of the oppressor. This imposition, in turn, may be made upon the oppressed population, which is allowed to remain, or upon the territory alone, after removal of the population and the colonization of the area by the oppressor’s own nationals.”

Lemkin’s initial definition has been debated since the 1940s and the definition continues to be debated today (like what would constitute cultural genocide). I bolded part of that because his writings directly impacted the creation of the UN definitions of genocide. The UN expanded the definition to include racial, ethnic, and religious as well as national (and some argue this should include political groups or gender/LGBTQ individuals as well). The UN defines genocide as:

Article II

In the present Convention, 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.

And that:

A physical element, which includes the following five acts, enumerated exhaustively:

· Killing members of the group

· Causing serious bodily or mental harm to members of the group

· Deliberately inflicting on the group conditions of life calculated to bring about its physical destruction in whole or in part

· Imposing measures intended to prevent births within the group

· Forcibly transferring children of the group to another group

However, the UN does state that acts of genocide can be done during peacetime or during a war and that (as Adam Jones writes) “One does not need to exterminate or seek to exterminate every last member of a designated group. In fact, one does not need to kill anyone at all to commit genocide! Inflicting “serious bodily or mental harm” qualifies, as does prevent births or transferring children between groups.”

So, with those definitions in mind and a little historical background, I think that one could argue that the Nanjing massacre was not just a war crime, but also genocide had it continued in a systematic manner across China. The occupying Japanese forces forcefully took over the city of Nanjing and continued to slaughter 200,000 or more Chinese men women and children and rape and tens of thousands of women and children in what Jones refers to as “genocidal rape" (likely because there was ethno-nationalistic hatred tied to the rapes of Chinese women). Acts of severe genital mutilation, disfigurement, dismemberment, and ultimately death were prominent. These Chinese people were targeted because of their nationality, but were killed in mass in an outburst of violence. Discussions among historians, sociologists, legal scholars etc sometimes differ over genocide in how society views, say, the Holocaust and Rwanda versus ACTS of genocide (like Nanjing, for example). The main point being a sustained commitment to “destroy, in whole or in part, a national, ethnical, racial or religious group” versus being an act of extreme violence located in one city.

So, to be considered genocide, by UN definitions, the act has to meet their specific definitions. That does not mean, however, that acts of genocide cannot occur (ethnic outbursts of violence targeting specific groups, for example) just that the UN may have different guidelines than Human Rights groups, sociologists or historians would use to define such atrocities. While the UN definition of genocide is useful in creating a working definition of genocide, I personally think it is more useful to study genocide in a more in-depth manner, working within definitions that sociologists or historians would use, as the UN (since it’s a legal institution) has weird ways of defining genocide when it is debating on if it should intervene in a country (mixed in with complex geopolitical constraints). As morbid and depressing as the topic is, it is an extremely interesting part of history that really combines several disciplines to understand how and why genocide occurs and how we define it, given that is still debated today.