Viking invasions started in around 790 AD. In 793 the monastery at Lindisfarne was attacked. Churches were an easy target for the vikings as they were built in remote locations and were poorly protected. Most importantly they contained ornaments made of silver and gold. The Vikings killed the monks and burnt the buildings. The monastery of Iona on the west coast of Scotland was attacked in 802 AD.
The number of attacks steadily grew and became an annual occurrence. At first the Vikings attacked in the summer and returned to their homelands for the winter. But soon the Vikings began to set-up permanent camps and stay in Britain over the winter months.
See the Viking Invasion Episode page for more information.