adjective

without defence or protection; totally vulnerable.

attacks on defenceless civilians

vulnerable,helpless,powerless,impotent,weak,frail,susceptible,easily hurt/wounded/damaged,impuissant,resistless,undefended,unprotected,unguarded,unfortified,unshielded,unarmed,without arms,without defences,assailable,open to attack,wide open,open,exposed,endangered,in danger,in peril,in jeopardy,at risk,insecure,pregnable