The Great Schism of 1054, which broke the Roman Catholic West from the Eastern Orthodox East.
It's the permanent division of the Christian church into two halves.
It was officially framed around a theological dispute (the 'Filioque' clause about the Holy Spirit), but underneath it was a geopolitical clash between the Roman pope's claim to rule everyone and the Eastern churches' preference for shared authority among patriarchs.
It shows how a fight dressed up as doctrine was fundamentally about who holds ultimate control.