Create a sapient creature. Figure out what it eats, and how it must go about gathering or acquiring its food. Because it is sapient, tool development for food acquisition will ensue, as it figures out more efficient ways to gather its current food. More complex tools will follow as your sapient becomes more efficient. Cities or wars may ensue.
Life is weapons escalation.
This holds as true for a plant's excreted toxic resins or a deer's better antler design or a skunk's more noxious skunk gland spray as it does for a human's Star Destroyer or Colt Peacemaker. Something, somewhere, will develop a way of getting around the defense to get at what it wants. Any species that expends its energy in weapons development while shortchanging eating, excreting, and reproducing is going to become extinct. Any weapon system that makes too big a drain on the creature's resources will cause its eventual extinction. Specialists are more efficient.