Before fusion reactors, before miniature hydrogen bombs, before violent chemical reactions, the Thaumic Drive was the only way to cross the space between stars. Still widespread, this technology, more a technique in actuality, is the practice of having a magus warp space around a craft, causing it to "fall" in any direction the magus chooses.
Only magi capable of manipulating gravity can create a thaumic drive, giving such people immense power and influence in early space-faring civilizations. This has the nasty effect of locking civilizations in "magocracy," rule by magi, for centuries. Magi protect their monopoly on movement by sabotaging scientific endeavors, stunting technological development in every field, including that of society. Civilizations that depend on thaumic drives tend to be authoritarian, with little room for non-magi in the decision making process.
While thaumic drives have existed since before history, as evidenced by many transplanted species across worlds, the first recorded thaumic drives date back to the First Aeternal Society. Humans used thaumic ships to colonize half a dozen worlds at this time. Although calling these early craft "ships" is charitable; they were more like pressured caskets with a magus inside. Later, much larger spacecraft came to dominate space. The largest required hundreds of magi to get up to reasonable interstellar speed and were mostly warships. Sedrua perfected the thaumic drive in the mid sixth millennium, training their magocrats to use them to devastating effect. They conquered three different civilizations on the cusp of fusion power just to violently suppress any development.
The development of the !Sen Fusion Reactor meant an end to the thaumic drive and magocracy in general. The Providence Union, creators of the technology, thoroughly conquered its thaumic-drive-using neighbors, prompting its rivals to either adopt fusion drives or face foreign rule. Fusion power has long since supplanted the thaumic drive, but remains widely in use in military applications such as stealth missions or shock forces.