Though domesticated, your cat still operates primarily by instinct. Ease introductions by respecting these differences, instead of bypassing them.
1. Be patient enough to devote weeks, even months, to the process. Go back a step or two at the first sign of discomfort in either cat, and proceed even more slowly from there.
2. Make surroundings more appealing. Extend your cats' territory vertically. Provide separate but equal food dishes, litterboxes, toys, beds and scratching posts. Employ pheromone-based products to comfort them. Lavish them with playtime and affection.
3. Consult a veterinarian or animal behaviorist if their interaction is consistently aggressive, even after you've repeated the introduction process.
4. Lower your expectations. Two cats living together may become best friends, but sometimes the best you can hope for is tolerance.