A few days ago Frank Buckles. the last surviving doughboy, died, moving the First World War out of the era of living memory and irrevocably into History.
In a couple of more years we'll be marking the centennial of the war's beginning, which could be considered the official celebration of the war's passage into the ranks of History.
Now History. of course, starts to be written moments after events happen, but so long as there are living eyewitnesses to events there's at least some check on the tricks of time and some hope of new information and perspectives arising. After the last witnesses pass on there is the occasional emergence of new archeological or documentary evidence, but even these will necessarily be interpreted by scholars with no first-hand knowledge of the events in question. The end result, in my view. is that once an event passes into History the chance to shape the narrative permanently passes. It's subject to revisionism, of course, but revisionism inherently is subject to revision itself and so whatever "truth" can be known about an event almost always must be established within the lifetimes of the witnesses.
This is one reason why phenomena such as the Lost Cause myth and Holocaust denial are so troubling when they occur because I think they're almost impossible to stamp out. If they can get established even while there are living witnesses to refute them, then later historians stand little chance of overcoming them. such myths serve powerful interests or they wouldn't arise in the first place.
The First World War didn't generate anything quite so noxious, but we still lose something when there's no witnesses left. How much and what we've lost we'll find out when 2014 rolls around.