Ten Years to Doomsday
by Chester Anderson and Michael Kurland
Published in 1964 by Pyramid Books.
Quoted from an Amazon review:
As the book begins, an evil race of aliens threatens the star-spanning
Terran Alliance. The aliens’ path to Earth leads through a
human-colonized world that seems particularly hapless. As we meet
them, the settlers on the planet Lyff seem a quiet people. They have a
rigid society which revolves around their king and petty nobility.
Even after thousands of years of civilization, the people of Lyff
haven’t passed beyond an agrarian lifestyle which barely provides
greater than subsistence living.
After their initial reconnaissance, the aliens plan be back in ten
years to conquer Lyff and then begin their implacable march through
the Terran empire. A stand needs to be taken on this small world, and
quickly. But the aliens have astounding technology. How can a tiny
planet like Lyff possibly defend itself?
Thankfully the Terrans have a plan: send a team of three scientists to
Lyff to help jumpstart the world’s technology. These men start with
the introduction of the telegraph but very quickly things begin to
take their own momentum and the colonials soon prove to be much more
sophisticated than the Terran colonizers expected. What at first seems
like indolence or a lack of ambition soon proves to provide a pattern
for technological innovation far beyond what anyone could have
expected. The arrogant Terrans learn there are more things in heaven
and Earth than are dreamt of in their philosophy.
The humans underestimate the Mother religion, and it turns out their holy book was actually about science and they thought it was about ethics. The humans introduction of new scientific concepts made them realize this. There's a scene at the end where the new ships each use a different method to launch, from different rocket fuels, interstellar drives or simply tractoring a star and letting it pull them off the planet. They do defeat the aliens. There is talk at the end of conquering Earth.