I often hear or read someone objecting to judging an historical person or act by present moral standards. Although there seems to be some element of reasonableness entangled in this objection, it's very problematic.
It is especially problematic as expressed. Technically, we cannot judge anyone or anything at all, except by whatever may be our present standards. If we judge historical people and acts differently from how we do present-day people and acts, it is exactly because our present standards incorporate a recognition of historical context.
I don't see that the real issue is historical context as such, but context more generally. If we are to make allowances for historical person or acts, it is because of what informed them and what did not inform them; and, similarly, acts by persons in some present-day contexts are very differently informed from acts by other persons in other present-day cultures. As L.P. Hartley usefully noted,
The past is a foreign country: they do things differently there.
Cultural relativism, in application to other places or to other times, is sensible when it warns one against presumption that one's own culture is doing things the only right way. (One's culture may be doing things a wrong way, or there may be other ways that are just as good.) But a cultural relativism that instead claims that something is automatically acceptable simply because it prevails in the culture of that place or prevailed in the culture of that time dissolves into nihilism because each person at each time and at each place is him- or herself a subculture.
And I think that some allowances should be made; but I think that too much allowance is often made.
For example, is the case against slavery now available really all that much better than the case that was available in America a few hundred years ago? Inverting that question, was the case against slavery available a few hundred years ago really much worse than the case available now? There is a sound argument, even to-day, for not waging war against slavery in the territories ruled by other states; and there may be a case for making treaties or even forming alliances with such states; but those are different practices from engaging in slavery or actively enabling slavery. Is there really a meaningfully better defense of the slavery of two hundred years ago than there would be of slavery now?
I don't think so.
Nor do I think so for a great deal else that I am told not to judge
by modern standards.