battle line

noun3

1. •
   The notional line on either side of which opposing forces are arrayed, and at which they meet in battle; a fortified line, short of the line where battle is expected, and behind which forces are arrayed; a forward line of infantry or other force, prepared for battle.

   • We did not catch them napping, but found a well-entrenched battle line confronting us. — 1912
2. •
   A line of battle.
3. •
   [usually plural] (figurative) Any of those parts of a contest which, as a result of strategic decisions on both sides, are most closely fought and will likely determine the contest's course and outcome.

   • There's battle lines being drawn / And nobody's right if everybody's wrong — 1966