وَلَقَد كانوا عاهَدُوا اللَّهَ مِن قَبلُ لا يُوَلّونَ الأَدبارَ ۚ وَكانَ عَهدُ اللَّهِ مَسئولًا

They had previously made a covenant with God that they wouldn’t turn their backs. The covenant with God must be answered for.