When I wrote a post about Project Daedalus, one of the assumptions built into the thought experiment was the existence of fusion power to move the spacecraft. For New Persia fusion power is part of the backstory.
To have interstellar travel and settle planets in other star systems, nuclear fusion reactor technology is necessary. The amount of energy needed to cross interstellar distances is immense. While there are some creative alternatives (solar sails and launching lasers), fusion power makes everything so much easier. Without it, the problems of interstellar travel are probably insurmountable.
In New Persia fusion power is not part of the present setting. What happened? Answers to come later in the series!
From Isaac Arthur, here is a good primer on the state of the art in fusion power. There is a discussion of the consequences of practical fusion reactors to human society.