Brethilian was part of Doriath during most of the Years of the Trees. When the Girdle of Melian was set after the First Battle, the Forest of Brethil was not included in it and was for the most part unsettled. However, King Thingol still held lordship over it and maintained a watch over the forest that made it a place of dread for any maurading orcs.
But it was not an orc who entered the forest that changed her life forever. Though she did not know it then, it was fate that stayed her hand. She disobeyed her King because her fea knew even than that she was fated to be Voronwë's wife.