Joel 2 - A locust invasion or human army?

An interesting question has come up in my study of Joel 2. Is Joel describing here 1) the same locust invasion described in Chapter 1, 2) a future locust invasion, or 3) an invasion by a human army? I have my own thoughts, but I'd love to hear yours. There are a lot of clues in the text itself without needing to consult commentaries or other outside sources.