The story
Campanilismo is Italian, built from campanile, bell tower, and the ending -ismo. The bell tower stands for the place you were born: the one you could see from your window. Campanilismo is attachment to that place, and Italian definitions are not gentle about it. They speak of an exaggerated, exclusive loyalty to your own town and its customs, which often turns into suspicion of the next town along.
Italy offers plenty of room for it. The rivalries run between neighbouring towns, between quarters of the same city, and between the contrade of Siena that race against each other in the Palio. In the 20th century football gave them a new stage. Someone who practises it is a campanilista.
English has an almost perfect structural match. Parochialism shares its root with parish, which reached English through Latin from a Greek word for neighbour. Both languages took a church landmark and turned it into a word for narrow local loyalty. Both can be harsh. In a book published in 1964 the statesman Alcide De Gasperi complained of the 'most narrow-minded' campanilismo. But the Italian word also covers the harmless kind: the festival, the football, the bakery that is better than the one in the next village.
Picture two hill towns facing each other across a valley in Umbria, each with its own tower. Their football teams have met for as long as anyone can remember, and each town will tell you who won the first match.