Beginning of the Common Era, also the starting point of the Imperial Calendar and the period of human history called the Age of Terra or sometimes the Age of Progress by Imperial historians. The Age of Terra is the time period spanning the 1st through 15th Millennia (1 - 15,001 AD) when humanity advanced from a pre-industrial Iron Age culture to become a star-spanning Fusion Age civilisation. The original reason for numbering the years in this way, based on the year of the birth of the ancient religious leader Jesus Christ, has long been forgotten by humanity, an overwhelming majority of whom at present profess belief in only one faith -- a deep belief in the God-Emperor of Mankind and his Imperial Cult.
A combination of an enormous boom in population and Terra's rapidly-diminishing natural resources in the 3rd Millennium leaves humanity's birth planet on the brink of anarchy. As these problems increase in severity, Mankind begins to look farther afield -- towards the stars. Mars is the first planet to be terraformed and settled. Colonial expeditions are initiated to explore space beyond the boundaries of the Sol System. These expeditions, all making use of sublight colony spacecraft known as Long March starships, meet with success. However, the Long March settlements furthest from Terra are forced to become increasingly self-sufficient due to the limitations of sublight interstellar travel.