Game of Thrones Season 6 Episode 05 The Door
Bran and the Three-Eyed Raven observe a vision of the Children of The Forest creating the Night’s King by impaling a captured soldier with a dragonglass dagger. Back in the cave, Bran confronts Leaf about creating the White Walkers. She explains that she had no choice, as they were at war with the First Men.
BRAN: It was you. You made the white walkers.
LEAF: We were at war. We were being slaughtered. Our sacred trees cut down. We needed to defend ourselves.
BRAN: From whom?
LEAF: From you. From men.
Source text: Wikipedia.