If there is a game called football the playing of which is already established in society (ignoring, already, that someone, at some point, created and established it as a game to play <insert eye roll here>):
Conservatism is continuing to play football as a thing society does — having a tradition of playing it, playing it at certain times or get-togethers, respecting and maintaining the rules as given.
Nationalism is having a favorite team and rabidly shouting “GO TEAM! GO TEAM! MY TEAM’S THE BEST, AND YOU ALL SUCK IF YOU DON’T AGREE!!!!!!!” no matter how much your team might actually suck, or how many women your favorite players have raped or abused, no matter how often they cheat or disrespect the rules of the game.
Conservatism is fine, so far as football might be a fine game. Conservatism when combined with modern, advanced, complex, sophisticated enlightenment philosophies such as constitutional government — if properly “conserving” the ideologies of new ideas like constitutional government even though those ideas may have come after the institution of football — will recognize that other people have a right to play other games like football (“soccer”) or basketball or tennis or chess or no games at all as part of their traditions. They recognize that accepting constitutional governments that presume equal, inalienable rights means they no longer have a right to do anything other than maintain their tradition of football for themselves, on themselves, by themselves and with anyone else who willingly joins them, and attempt to promote their game of football, and their team to others.
A form of sportsmanlike loyalty or fandom that could be construed as “nationalism” is okay, in a world that inevitably requires competition, and in that world where one has simultaneously proclaimed constitutional government and inalienable rights. But that fandom is far milder and might, at extremes, consist of mischievous things like shit-talking other teams and hazing other teams’ players, toilet-papering houses, etc. It is unacceptable, extreme nationalism and no longer the natural loyalty that simple animals feel for their tribe when — in the simultaneous context of self-proclaimed enlightenment ideals — they play “pranks” to shut the other team out of the game, slander players for things unrelated to the game to try to tear them down, try to destroy or shut-down other people’s games, or lack respect for the choice of any given person to change teams, change games. All the things that out-of-proportion, zealous, scared people do beyond simply playing football as well as possible, by its established rules, to show within the playing of the game that they can play well and are “better” than the other teams, let alone the ideals of sportsmanship that would be applied by valid conservatives at this stage of the development of society even if they might be “the best” players, the best team.