Complex sentence: In the 860s, instead of raids, the Danes mounted a full scale invasion, and in 865 arrived an enlarged army that the Anglo-Saxons described as the Great Heathen Army.
Split into simpler sentences: In the 860s, instead of raids, the Danes mounted a full scale invasion. In 865 an enlarged army arrived that the Anglo-Saxons described as the Great Heathen Army.