Things that have happened in the past, like yesterday and even further back than that. Always written by the victors who get to make up stuff. This is done to cover up all the bad things they did to win, make their country look better, to cover up incompetent commanders (i.e. Haig in WW1) and any horrible kit (i.e. Sherman in WW2). All the bad stuff the losers did will be perpetually stuck under a microscope and used as a scapegoat forever.
See also: Pre-History