WWII is, in many ways, the demarcation point of the modern era. Almost every facet of life was changed. Women entered the workforce, agriculture and industry were more or less entirely rewritten.

In terms of genocide, it was the first one to use computers. It used systematic, organizational principles similar to what a business might use. It created a set of processes both to carry it out and to make the population accept and even participate in the unspeakable.

I would say that a pre-modern genocide would be done that was committed by roving bands of soldiers, rather than a bureaucracy.

A bit of research reveals I was wrong in my assumptions, though, and the Armenian genocide certainly counts by that metric.

http://en.wikipedia.org/wiki/Armenian_Genocide

That was the exact one I was thinking of. I really think you should look at the source that I linked to though. IBM knew what it was participating in and has since apologized (though never paid for it).

Perhaps it's `first' as in `most on people's mind'?