The Terro-Human Future History is Piper's detailed account of the next 6000 years of human history. 1942, the year the first fission reactor was constructed, is defined as the year 1 A.E. (Atomic Era). In 1973, a nuclear war devastates the planet, eventually laying the groundwork for the emergence of a Terran Federation, once humanity goes into space and develops antigravity technology.
The story «The Edge of the Knife» (collected in Empire) occurs slightly before the war, and involves a man who sees flashes of the future. It links many key elements of Piper's series. (Summary from Wikipedia)