And what on earth were they doing?
I witnessed this on the sidewalk this morning. It didn't look like a battle, although the close-ups show plenty of mandible-to-mandible contact. I think it was some sort of information-exchange extravaganza. According to a review1 in the current issue of Current Biology, ants communicate with each other using "multiple pheromones, displays, contacts and sounds".
Whatever it was, to my feeble human senses it looked like a totally chaotic event. But for the ants, and I am quite certain of that, this was probably a perfectly ordered activity completely within the boundaries of ant logic.
1. Duncan E. Jackson and Francis L.W. Ratnieks. Communication in ants. Current Biology, Vol 16, R570-R574, 08 August 2006. pdf