Complex sentence: When his father died in 1806, he succeeded in the duchy of Saxe-Coburg-Saalfeld as "Ernest III", but could not take over the formal government of his lands, because the duchy was occupied by Napoleonic troops and was under French administration.
Split into simpler sentences: When his father died in 1806, he succeeded in the duchy of Saxe-Coburg-Saalfeld as "Ernest III". However, he could not take over the formal government of his lands, because the duchy was occupied by Napoleonic troops and was under French administration.