Towards the end of the twenty-first-century humans were still bound to earth, a lack of energy and easy means to produce it meant that only a few nation-states possessed enough to make life comfortable for their people, while the rest could only dream, and fight for the ability to duplicate that life. As a result warfare was a constant feature and it felt too many that war had become a way of life, powerful nation-states fought to maintain their supplies and life, while poorer nations fought to better their position (at least that was often what the rallying cool of leaders of all kinds.)
Then a Sino-European corporation called TransPac Energy Research Conglomerate – an expert in geothermal and nuclear fission reactors had a breakthrough at a test facility leading to the creation of the first viable fusion reactor complex. Several years of further research and development led to the construction of a small number of test reactors in China and North America, results from which fuelled massive investment. By the mid-twenty-second-century fusion power had become the de facto energy source for modern nation-states.
With energy and by extension water and fuel issues solved corporations and nation-states looked to the solar system to solve the raw material problems that couldn’t immediately be solved. As time went on reactors became small enough to deploy on orbital construction facilities and shipped to create settlements viable on other worlds.
The first areas to be exploited were asteroid belts and the creation of colonies on Mars and the Moon, by the end of the twenty-third century humans have a presence on or around everything orbiting the Sun.