The Taira clan possessed overwhelmingly superior power over the Minamoto clan but was subsequently made vulnerable (by some viewpoints) when they were surprisingly attacked by the Minamoto clan (there was limited fighting during the battle), and the Taira clan then lost the battle, suffering a massive defeat.
圧倒的に優勢な平氏は（見方によっては）ほとんど武装解除に近い状態にあったところを源氏に襲われて（まともな合戦にもならず）大敗を喫したということになる。