Spoiler alert!

Edit: the book is "The Road not Taken"

There's a scifi story about a civilization stumbling upon how to achieve FTL travel. In the story, the tech is at the same time very simple and very unexpected. Anyways, they go explore the galaxy and invade and conquer with their primitive ships, which are little more than tin buckets. Their weapons technology is on the flintlock gun level.

(A tragic kind of) hilarity ensues when they stumble upon Earth with its completely unexpected, incredibly advanced weaponry. IIRC in the story most civilizations find FTL travel pretty early. Just Earth didn't happen upon it and instead had time to develop advanced weaponry, computers, etc.

Ooh, thanks for this. Apparently it's here, and is short enough to read in a few minutes, so I know what I'm doing!

https://www.eyeofmidas.com/scifi/Turtledove_RoadNotTaken.pdf