Warhead

An away team finds a crashed device whose artificial intelligence can talk, and the Doctor insists the crew help it. It turns out to be a highly advanced missile carrying a powerful warhead, which has lost its memory. While Torres and Harry Kim try to disarm it, it takes control of the Doctor's program and demands to be taken to its target, threatening to blow up the ship.
Kim tries to reason with the warhead and finds evidence that its war has ended and that its mission was cancelled, though the recall order is protected by a code that is almost impossible to read. Dozens of its sister weapons then arrive, ignoring the order to stand down. Realising what it must do, the warhead returns to its own casing and joins them, detonating to destroy them all. The fifth-season episode gives Harry Kim a central role as a negotiator.