When the Earth is invaded by aliens, they are swiftly defeated by humans, because the invaders’ weapons are no more technological than gun powder, their communications are limited to shouting orders to one another, and their armor is iron plate. In other words, the level of the invaders’ technology did not seem to match their ability to travel to distant star systems.
It turns out that the secret of FTL was so simple that it was typically discovered about the same time as the invention of black powder. This technique was somehow never discovered by humans. Being stuck on Earth, humans continued to develop new technologies to conquer each other in a never-ending arms race that no alien species ever needed to do, because easy access to space travel did not require them to.
What is this story?
Thanks in advance!