Hello. Half-life: Opposing Force sticks out as an expansion pack, aside from being superb, it also gave us an alternate viewpoint of the same events from the point of view of the marines.
Resident Evil 2 slightly pre-dates it. While in Resident Evil 1 the swap in characters merely changed the characters abilities, Resident Evil 2 featured an alternate point of view with different starting points, enemies bosses, story and a zapping system.
These two stand out, but were there other earlier "alternate perspective" games? What's the earliest/first-example?