The Two Crowns, Silver and Gold
Winter War of 1149
The Winter War of 1149 was a full scale war between the mice of the Territories and the weasels of Darkheather. The war itself began in the Winter of 1149 when the weasel lord Rampaul led his army of mustelids though the Territories, destroying several settlements in the process. Forced to fight back, the young Matriarch Gwendolyn waged war on the weasels. They were driven back, but at a high cost; many Guardmice and common mice were killed in the process.
From Mouse Guard Wiki