David Petersen announced the news at the Archaia panel.

Source: Bleeding Cool

During Archaia's panel at SDCC, David Petersen announced that the next volume of Mouse Guard would be a prequel comic entitled Mouse Guard: The Weasel War of 1149. Fans of the series will recall that the war has been referenced in the first two volumes of the book, and that the mice were led by a young Gwendolyn in the assualt. The Black Axe, Celanawe, claimed that he participated in the battle as well.

Petersen also announced a third volume of the anthology series Legends of the Guard, and that he's already contacted Fables creator Mark Buckingham and Skottie Young for stories.

Here's a promotional print of Mouse Guard: The Weasel War of 1149, courtesy of Bleeding Cool: