system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why did we Americans name our sport "Football" when there was already a sport called that with entirely different rules?
gpt
Because your premise is faulty. There is no single game whose English name is "football". Football is a category of games played on foot and using a ball (in opposition to something like polo that uses a ball but is played on horseback).  Games within the football category include Association Football, American/Gridiron Football, Australian Football, and Rugby Football.  Also, most of these had their rules codified in the 1860s, so there's not even much if a which came first. 

The form of football that became popular in Europe is Association Football, this becomes more evident when you look at what FIFA stands for (federation Internationale de *football association*).

Fun fact: the term "soccer" to refer to association football comes from England. In the 19th century students at Oxford had a tendency to create slang names by adding an "-er" to the end, thus Rugby became Rugger and Association Football became Assoccer and then finally Soccer. 


Tl;dr: there is no single game whose English name is football. Your premise is faulty.