“Football games are won and lost in the trenches” (football adage)

Football’s “trenches” are the offensive and defensive lines. “Football games are won and lost in the trenches” is a saying that borrows from the military use.
 
“There is a gridiron adage that football, like war, is won in the trenches” was cited in print in 1951.
 
 
28 September 1951, Naugatuck (CT) News, “Yale-Middle Tilt Will Draw 50,000 To Bowl Tomorrow,” pg. 6, col. 6:
There is a gridiron adage that football, like war, is won in the trenches.
   
Sports Illustrated
October 15, 1956
Football: Third Week
The crowds were growing larger despite some nasty weather, and the games were starting to count—toward league and national titles
James Atwater, Martin Kane , Willard C. Rappleye, Lee Griggs
MICHIGAN STATE 9
MICHIGAN 0
When Michigan plays archrival Michigan State—as Fritz Crisler, Michigan’s athletic director, noted on the eve of the big game—it’s civil war. Last Saturday each team brought along its blitzkrieg offense, but the game was fought and won up in the trenches where the linemen wage their savage, static combat.
 
Google News Archive
6 November 1970, Tuscaloosa (AL) News, “In Tide-Tiger Tilt: Linemen To Decide Outcome?” by Ed Darling, pg. 7, col. 3:
Week in and week out you hear talk about football games being won in the trenches.
 
Google Books
1 December 1972, Life magazine, pg. 61:
This game is won in the trenches.
 
Google News Archive
4 October 1974, Victoria (TX) Advocate, “Stroman Visits Lamar” by Cecil Parker, pg. 1B, col. 1:
“You’ve heard of games being won or lost in the trenches? I don’t think they dig trenches big enough for the Raiders.”
(Lamar head coach Bill Lucky—ed.)
 
Google Books
Offensive Football Strategies
American Football Coaches Association.
Champaign, IL: Human Kinetics
2000
Pg. 108:
Blocking the Wishbone
By Jimmy Sharpe
The game is won in the trenches and, regardless of what offensive formation you use or believe in, that 18 inches of combat zone determines who will win the game.