Complex sentence: Around 722, a Muslim military expedition was sent into the north in late summer to suppress a rebellion led by Pelagius of Asturias (Pelayo in Spanish), and overran much of Pelayo's territory, forcing him to retreat deep into the mountains.
Split into simpler sentences: Around 722, a Muslim military expedition was sent into the north in late summer to suppress a rebellion led by Pelagius of Asturias (Pelayo in Spanish). The Muslim forces overran much of Pelayo's territory, forcing him to retreat deep into the mountains.