The Great Schism of 1054, which broke the Roman Catholic West from the Eastern Orthodox East[1].
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.