Complex sentence: Marshall Masséna began his campaign with his army (l'Armee de Portugal) at 65,000 strong, by the time he reached Torres Vedras he had 61,000 men (after losing 4,000 at the Battle of Buçaco).
Split into simpler sentences: Marshall Masséna began his campaign with his army (l'Armee de Portugal) at 65,000 strong. By the time he reached Torres Vedras, he had 61,000 men (after losing 4,000 at the Battle of Buçaco).