I'm looking for a story where humans fight against aliens. The plot is, the aliens discovered gravitational space travel early in their history, and although being very advanced in space travel technology, the aliens are really late against human warfare technology.
The Road Not Taken by Harry Turtledove.
Space travel is super simple, but somehow humans never discovered it. The aliens have never developed the alternative technologies we have, relying on conquest of primitive races with their space ships made of wood instead.
When they land on Earth, they get utterly defeated by our superior weapons and humans go on to rule the galaxy.