The book is written in the 1980s. The storyline is about an early 20th century level technology planet getting visited by a spaceship which causes quite a stir. The visitors start helping to consolidate all of the countries on the planet. Everyone thinks this is good but some people accidentally find out they're actually being conquered by using the planets own people to do the fighting.
They also find out that when all the planet's countries are defeated and under the control of a single government, the planet will be colonized by a space federation that is reclaiming long lost planets.
Through trickery they learn that if they can somehow get space travel they wouldn't be colonized and would have rights and a higher place in the federation. They start sneaking around trying to make a spaceship with their early 20th century tech.