"The Aztec nobles rally, giving you approximately 1 Arrow Knight for every 2 minutes of game length, up to 30 minutes."—In-game description
War Song is a Big Button technology in Age of Empires III: The WarChiefs that is unique to the Aztecs and can be researched at the Nobles' Hut. Once researched, one Arrow Knight for every two minutes the game has passed, up to 30 minutes (for a total of 15 Arrow Knights), will appear in the Home City shipment point.
War Song is an easy way of getting a large force of anti-building and anti-artillery troops.