Why did Pope Innocent III excommunicate King John of England?

Pope Innocent III excommunicated King John of England in November 1209 primarily due to a dispute over the appointment of the Archbishop of Canterbury.1

Following the death of Hubert Walter, a contested election for the see occurred. Innocent III rejected the candidate favored by King John and instead secured the election of Stephen Langton in 1206. King John, asserting the traditional rights of the English crown regarding episcopal elections, refused to accept Langton’s appointment.1

In response to John's defiance, the Pope initially placed England under an interdict in 1208 before proceeding to excommunicate the King the following year. The conflict persisted until 1213, when John finally agreed to accept Langton and the Pope’s terms, eventually surrendering his kingdom to the papacy as a vassal to resolve the standoff.1