Reciting the creed against Arianism in 589, the Third Council of Toledo added one word to the clause on the Spirit: filioque, and the Son. It was a local defence of the Son's divinity, not a claim against the East. But it spread with the Frankish liturgy, and by 809 Leo III was affirming the doctrine while refusing the interpolation — he had the creed engraved without the word on silver shields at St Peter's. Rome sang it anyway from 1014, and it is the first article of the charge sheet in 1054.
East and West: The Making of a Rift in the Church · ch. 2