One proposed method of interstellar warfare involves nanotechnology. You launch millions of tiny machines at near the speed of light. These could be the size of a pin or smaller. A fraction of them survive and begin to self-replicate and take over their planet. Do this for every solar system they inhabit.
Couldn't they defeat it with their own nanotech? Well possibly. Perhaps the winning civilization is just slightly more advanced and has an advantage. Perhaps they slip in somewhere undetected and have time to mess things up before they can be stopped. Perhaps stopping nanobots from self-replicating is just harder to do than replicating.
Who really knows what technology is possible. An 18th century person never would have predicted machine guns and planes. A 19th century person couldn't have anticipated nuclear weapons. Few 20th century people probably even heard of nanotechnology. We can't guess a century of technological progress, let alone millions.
Couldn't they defeat it with their own nanotech? Well possibly. Perhaps the winning civilization is just slightly more advanced and has an advantage. Perhaps they slip in somewhere undetected and have time to mess things up before they can be stopped. Perhaps stopping nanobots from self-replicating is just harder to do than replicating.
Who really knows what technology is possible. An 18th century person never would have predicted machine guns and planes. A 19th century person couldn't have anticipated nuclear weapons. Few 20th century people probably even heard of nanotechnology. We can't guess a century of technological progress, let alone millions.