For the sake of a purely historical debate I want to ask people at what point do they think that the fate of certain empires and cultures was sealed and what they think the future would have looked like if these people had held onto their independance.

For example the continental Celts, at what point in history were they destined to be absorbed into Roman, Germanic and Dacian cultures? Alesia? The failure of Carthage in the second Punic War? Or even when Brennus simply sacked Rome rather than destroy it? Or were they doomed anyway?

Or for the Hellenic fans (numerous as you are) when was the fate of the Seleucids sealed? Magnesia? The loss of Babylon to Parthia? Or when Laenas drew the circle in the sand around Antiochus IV?