According to 9th- and 10th-century sources, the Gaelic kingdom of Dál Riata was founded on the west coast of Scotland in the 6th century. In the following century, the Irish missionary Columba founded a monastery on Iona and introduced the previously pagan Scoti and pagan Picts to Celtic Christianity. Following England's Gregorian mission, the Pictish king Nechtan chose to abolish most Celtic practices in favour of the Roman rite, restricting Gaelic influence on his kingdom and avoiding war with Anglian Northumbria. Towards the end of the 8th century, the Viking invasions began. Successive defeats by the Norse forced the Picts and Gaels to cease their historic hostility to each other and to unite in the 9th century, forming the Kingdom of Scotland.