Any of probably millions of historical wars that were fought over issues that don't affect people anymore? There may well have been very visibly good and bad sides and principles in the conquest of Sumer in 2271 BCE, but that doesn't mean we should be going to that border to fight over it now. Throughout history, thousands of wars were won, and atrocities done by evil people, and yes, 'they were unfortunate things that happened in the past', that sucks, but you can't chase down every one of those wars and keep fighting it just for the sake of principle.

At some point people had to get over those grievances and focus on fairness and humane treatment of people today. Again, that doesn't mean forgetting the rights of people who are alive now and still affected by past injustice.