battle line

plural battle lines · alternative battle-line

1. governmentThe 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 February–July, Edgar Rice Burroughs, “Under the Moons of Mars”, in The All-Story, New York, N.Y.: Frank A. Munsey Co., →OCLC; republished as “Through Carnage to Joy”, in A Princess of Mars, Chicago, Ill.: A[lexander] C[aldwell] McClurg & Co., October 1917, →OCLC, page 310

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   "You say he is in Armstrong's rear, do you? Damn him! That's just where I have been trying to get him all day, and I'll be in his rear directly. Face your battle lines about! Armstrong, push your skirmishers forward -- crowd 'em both ways! I'm going to Starnes. You'll hear from me in about five minutes!"

   1894, Dabney H. Maury, Recollections of a Virginian in the Mexican, Indian, and Civil Wars, section 17

   That is the reason why I have asked you to take out and spread before you a map of the whole earth, and to follow with me the references which I shall make to the world-encircling battle lines of this war.

   1942 February 23, Franklin D. Roosevelt, Roosevelt's Fireside Chat

   1965, Hugh M. Cole, The Ardennes: Battle of the Bulge, Chapter XX: The XII Corps Attacks the Southern Shoulder, The German intelligence staffs again agreed that there was no immediate threat to the Seventh Army, and that the westward advance by the Fifth Panzer Army would necessarily force the Americans to strengthen the battle line there and prohibit any thrust into the deep southern flank.

2. governmentA line of battle.
3. figurativelyplural normallyAny 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 December, Stephen Stills, “For What It's Worth”performed by Buffalo Springfield

   2 more examples

   Mancini's side only showed enough ambition when it was too late and by then battle lines were drawn as James Collins and former City captain Richard Dunne fought a magnificent rearguard action at the heart of Villa's defence.

   2011 January 22, Phil McNulty, “Aston Villa 1 - 0 Man City”, in BBC

   Especially that absolute territory peeking out from between her socks and skirt, like a battle line between innocence and temptation...

   2018, Ichiro Sakaki, Sasha McGlynn, Outbreak Company, volume 4