Complex sentence: Lambert, a former count of Nantes, after devastating Anjou in concert with Nominoé, duke of Brittany, had, by the end of the year 851, succeeded in occupying all the western part as far as the Mayenne.
Split into simpler sentences: Lambert, a former count of Nantes, devasted Anjou in concert with Nominoé, duke of Brittany. By the end of the year 851, he had succeeded in occupying all the western part as far as the Mayenne.