The problem with David Starkey's comment that 'slavery is not genocide' is, of course, mainly to do with the racism. But additionally it shows that he doesn't understand the nature of genocide and hence what constitutes genocide.

Article 2. of the UN genocide convention defines genocide as ""intent to destroy, in whole or in part, a national, ethnical, racial or religious group, as such". The 'as such' is critical, because it is about obliterating the group as a group.

Certainly killing people is one means of achieving that, but it is not the only way. One can destroy groups by stopping childbirth, transferring children to other. groups, denying the practice of a culture and language and so on. And different genocides take different forms.

so slavery was not about killing every black person, it was about stripping them of their collective and individual personhood, turning them into chattels that could be exploited until they broke.

Of course, in doing so, countless people died. But here, it was more that that the slave traders were indifferent to mass murder than that this was the end they set out to achieve.

By contrast the holocaust did set out to murder every single jewish person. It was not primairily about exploiting jewish labour. Certainly jewish labour was exploited along the way. But the Nazis exterminated communities even when they remained productive.

Tragically, this is what many jewish ghetto leaders, like Rumkowski in Lodz, never understood. They thought that if they made the community useful to the Nazis they would be saved. They were wrong.

So the point is that these are qualitatively distinct forms of genocide. It isn't that one is 'worse' than the other. It is that they are different and, in confusing one with the other, one is led into potentially disastrous errors.

Moreover the numbers of people who die and the numbers who survive has nothing to do with whether an event is a genocide or not. And to suggest that is does shows a remarkable ignorance of basic history from one who poses as an eminent historian.