firing line

firing line
    n 1: the line from which soldiers deliver fire
    2: the most advanced and responsible group in an activity; "the
       firing line is where the action is"
    

24 Moby Thesaurus words for "firing line":
      DMZ, aceldama, battle line, battle site, battlefield, battleground,
      combat area, combat zone, enemy line, field, field of battle,
      field of blood, front line, killing ground, landing beach, line,
      line of battle, seat of war, shambles, the front, theater,
      theater of operations, theater of war, zone of communications