Football's substitution rules have changed as often and altered the game as profoundly as almost any rules category. Only the transition from rugby's scrummage to the scrimmage, the addition of the system of downs, and legalizing the forward pass have had a more significant impact. Still, while the game's substitution rules changed numerous times before the 1940s, they did not reshape football until two-platoon football became legal. In the case of college football, this did not occur until two-platoon football became legal for the second time.
Like rugby, early football did not allow substitutes by convention, not rule. Football's first set of rules in 1876 did not mention substitutions, stating only that "the number of players shall be limited to fifteen upon a side." That changed in 1882 when substitutes were allowed to replace disqualified or injured players and again in 1897 when substitutions could enter the game at the captain's discretion. The 1905 rules allowed substitutions at any time, assuming the incoming player reported to the referee before lining up, and also formalized the convention that substituted players could not return to the game.
Limited or no substitution was the norm in the sporting world back then. For example, substituted baseball players could not return to the game. Ice hockey prohibited substitutes other than replacing injured or disqualified players. High-level soccer first allowed subbing in 1954, and rugby still does not have subs (other than blood replacements).
Even when football began allowing limited substitutions, the manner differed from today. Most substitutions occurred late in games when the starters tired. Substitutions also occurred when strong teams started their reserves in early season games against lesser opponents to save their top players for the season-ending big games. Squads that met unexpectedly strong resistance in those games sometimes sent out first-string players to mop up and ensure the victory. In rare circumstances, coaches sent in reserve players who were effective kickers, but even those substitutes had to play a regular position after that.
A side effect of players being unable to return to the game was that dazed or otherwise injured players often stayed on the field, taking time to recover before returning to play.
Recognizing the rule created a safety threat, a 1910 rule change allowed substituted players to return to the game at the beginning of a subsequent quarter. Coaches leveraged the rule after WWI by using "shock troops," which initially referred to coaches swapping small groups of substitutes, such as the four backs. It gained its current meaning in 1924 when Knute Rockne started his second unit in the season opener, planning to tire his opponent before inserting the first team. Other coaches with deep rosters followed Rockne's lead. Regardless of the substitution pattern, whoever was in the game played offense, defense, and kicking plays. The result was that players of the period needed the general athleticism and stamina to play offense and defense for up to sixty minutes. Teams split their practice time accordingly and had smaller rosters, fewer coaches, and less complex schemes than today.
Football might not have flipped its substitution rules if not for WWII. America began mobilizing for war in 1940, and with draftees and volunteers leaving campuses, concerns arose about the depth of college football rosters. To allow coaches to substitute for an injured or tired starter while also allowing them to return to the game, the 1941 rules committee approved unlimited substitutions, meaning players could enter and reenter the game whenever the ball was dead. Intended as a temporary rule, the rules committee and everyone else expected substitutes to enter one or a few at a time as short-term relief, and that is how coaches applied the rule until Fritz Crisler gambled on a new approach against a superior opponent.
Crisler was Michigan's coach in 1941 when he presided over the rules meeting that approved unlimited substitutions. Still, even he did not appreciate the door opened by the new rule. In 1945, however, his squad was filled with freshmen not yet of draft age and others designated as 4-Fs, or physically unfit for the armed forces. In the week leading up to their game with West Point and its future Heisman Trophy winners -Doc Blanchard and Glen Davis- Crisler created separate offensive and defensive units, swapping them with each change of possession. (A few top Wolverines played both ways.) Although outgunned, Michigan lost 28-7, Crisler's strategy caught other coaches' attention, and some copied his approach.
Red Blaik was Army's coach, and while I wrote in the past that Blaik coined the term "two-platoon football," recent research shows that was not the case. While "platoon" is a military term, "two-platoon" described the rotating shifts in police and fire departments beginning in the 1870s. "Two platoon" leaked over to football as an alternative term for shock troops used by Frank Leahy's 1940 Boston College and Buff Donelli's 1942 Duquesne teams before transitioning again to describe substituting full offensive and defensive units throughout the game.
Nevertheless, Blaik helped popularize two-platoon football (and the term) because the West Point cadets' busy schedule allowed only seventy-five minutes of practice each day. Platooning allowed Army players to focus on offense or defense, and alleviating that handicap helped Army sustain their 32-game win streak that ended during the 1947 season. Nevertheless, many coaches and fans derided two-platoon football because it ran counter to the long-held ideal of the all-around athlete and sixty-minute man. Others argued that the waves of players entering and exiting the field confused fans. Some fans agreed, including those who booed Army when they platooned against Stanford at Yankee Stadium in 1948.
The more significant challenge to two-platoon football in college football was that it required larger rosters and more coaches, increasing costs. Many colleges that suspended football during WWII struggled to regain their competitiveness after the war, and the increased cost of two-platoon football added to their burden. After fifty colleges dropped football for cost reasons in the early 1950s, the NCAA membership eliminated free substitution for the 1953 season.
Meanwhile, the NFL had liberalized its substitution rules during WWII by allowing three subs whenever the ball was dead or by mass substitutions between quarters. The NFL moved to unlimited substitutions in 1950 and has retained the rule ever since. The NFL's increased popularity in the 1950s provided the cash to expand rosters and protect their star quarterbacks from potential injury playing defense. Separate offensive and defensive units became the norm, leading to specialist offensive and defensive coaches who focused on one side of the ball, creating and teaching more specialized techniques and concepts to players who could absorb those details. It is not a coincidence that the term "special teams" emerged in the mid-1950s as NFL teams increasingly mixed starters and substitutes on specialist punting and kicking units.
Under pressure to keep up with the innovative professional league, college football tweaked its substitution rules nearly every season from 1953 until the mid-1960s in a slow march back to unlimited substitution. College coaches saw the benefits of specialization in the pro game and adopted elements into their systems. In addition, two-platoon football ensured players were less tired, gave playing time to those with skills suited to one side of the ball or the other, and broadly, gave playing time to more players. Still, there were challenges in balancing the demands of those supporting more liberal versus conservative substitutions rules, resulting in a few inconsistent or incoherent rules in its march back to two-platoon football. For example, the 1958 rules allowed each player to substitute into the game twice per quarter, which required players to check in with the officials who monitored each player's status. Lines of players checking into games became a common sight that season. Such inanities disappeared when unlimited substitution returned to the college game in the mid-1960s.
While early players largely remained on the field for the entire game, football ultimately took the path of substitution and specialization, which dramatically changed the game, the players, and coaches. The specialization of players and coaches has walked hand-in-hand with the game's increased complexity and has literally reshaped the players. Today, most high-level players would be unable to play the ninety-, seventy-, or sixty-minute game of one-hundred-plus years ago. Current selection and training processes assume the players will be involved on only one side of the ball, and their skill sets and physiques reflect that assumption. Few of today's offensive ends could play defensive end as their grandfathers did. Few traditional quarterbacks would be effective safeties. Most interior linemen could not maintain adequate play on both sides of the ball while covering kicks throughout a game. None of this is a criticism of the current game. Instead, it simply points to how far football has evolved from its roots as a single-platoon game. Ironically, the move to two-platoon football was an unintended consequence of a rule that addressed a short-term problem. What might the game look like today if the substitution rule had not changed in 1941 or if Fritz Crisler had not developed his scheme to face a superior opponent?
Feel free to comment on how football might have developed differently had the game not moved to two-platoon football.
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This article is incorrect. Substitution was added to the Laws of the Game for association football (soccer) in 1958, not 1965. The 1965-66 season was when the Football League in England first allowed substitutions, though they had already been allowed in other countries.
You can find more details here: https://www.theguardian.com/football/the-agony-and-the-ecstasy/2020/jul/15/starters-five-brief-history-substitutions-in-football