Time to use my poli sci degree and make my professors proud!

Genocide, as a defined concept, and as a legal term, is a very new word and idea. It dates back to the 1930s and a Polish-Jewish lawyer named Raphael Lempkin, who described what we now call the Armenian genocide as a “crime of barbarity”. He was trying to get the League of Nations to accept this term as a new crime under international law for extreme crimes like that of the Armenian genocide, as no concrete legal term--nor even a single word in any language--for such acts of violence. While he was unsuccessful with the League of Nations, his efforts later materialized as the term we now use: genocide. Genocide is a legal term, with its basis in the Convention on the Prevention and Punishment of the Crime of Genocide (CPPCG), or sometimes called UN Genocide Convention (UNGC). I usually use the latter term and abbreviation, including in the rest of this answer.

So, with that in mind, we can somewhat answer your titular question: yes, it is a modern invention… as a word and a legal term recognized by international law/multiple states in the world. However, if we look back beyond the 1930s and Mr. Lempkin, we can retroactively apply the term to cases of violence in history which meet the definition of genocide. The creators of the UNGC acknowledged this, writing in the preamble of the convention:

Recognizing that at all periods of history genocide has inflicted great losses on humanity.

So, the creator of the term recognized acts of violence as genocide before he coined the term, at least as far back as the 1915 Armenian genocide; he also described other acts of genocide in history, including the crusade against the Cathar people in France in the 1200s. We also see the people who codified the term genocide into law believed genocide occurred before the Holocaust (which was the reason the UNGC’s was created), at least.

I argue the term can correctly be applied to various historical acts of violence. By examining the definition as laid out in the UNGC, by Mr. Lempkin, and by comparing historical events to modern ones wherein genocide is applied, I think genocide is not a new act by humans. Since WWII, several cases of genocide have been argued by the UN, human rights groups, governments, etc. These include, but are not limited to, violence in the former Yugoslavia and in Rwanda in the 1990s. I list these two cases specifically since the UN created special tribunals for each case to hear cases on and charge individuals with genocide and other crimes against humanity (CAH). These two examples are current real world applications of the term genocide and I will compare historical examples to them. The special tribunals created are the International Criminal Tribunal for the Former Yugoslavia (ICTY) and the International Criminal Tribunal for Rwanda (ICTR), which later led to the International Criminal Court (ICC), which is now the UN’s permanent judicial body for prosecuting persons on the crime of genocide, as well as other CAH. Judges and lawyers have argued, successfully, that genocide occurred in those two cases, which is why I use them for my historical comparison.

In both cases, a large amount of people belonging to one or more groups identified in the UNGC were specifically targeted and killed because they belonged to a specific group. In Yugoslavia, it has been ruled by the ICTY that the slaughter of (estimated) 8000 Bosniak men and boys was genocide; in Rwanda, the ICTR ruled the four months-long murder of (estimated) as many as a million Tutsi Rwandans (and Hutu sympathizers) was genocide. These are two widely different cases in terms of numbers, but they are similar by the nature of the violence: specific targeting of people belonging to a clearly identifiable group. So, the question everyone has been waiting for me to address: WHAT ABOUT THE PRE-WWI EXAMPLES?! I’m finally there!

Ben Kiernan described Carthage as “the first genocide”, so he would agree with you. The tricky thing about genocide is there is no set definition on how many people must be killed before it is genocide. The UNGC says,

...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…

So, in the simplest reading, the Romans committed genocide by destroying Carthage as they demonstrated through their actions the intent to destroy, in whole or in part, the Carthaginian people. The Carthaginians were a national group, at least. I don’t know enough about them to say if they could be considered an ethnic and/or religious group, under the UNGC, but I will hazard a guess and say they could qualify as either or both. I don't know if there are any reliable numbers (any classical historians in the house?) on how many were killed, but the estimates I've seen number as high 200,000. Compared to the former Yugoslavia, where some 8,000 Bosniaks were killed at Srebrenica and were specifically targeted because they were Bosniaks, we can reasonably conclude that was genocide.

Other scholars describe the Mongols as having committed numerous acts of genocide. It’s hard to say what is and isn’t genocide since the legal definition is vague and open, but I think you can argue a lot of massacres in history were genocide.

I hope this has been helpful! I will try to answer any further questions anyone has.

I know little about this, but as a follow up question I'd like to ask whether the idea of genocide presupposes a concept of ethnicity, which I'd argue is a pretty modern invention?