Complex sentence: As Dubourdieu had planned, this detour opened Lissa to attack; landing on the island with overwhelming force on October 21, the French seized the shipping in the harbour but were unable to find the island's garrison, which had retreated to the mountains.
Split into simpler sentences: As Dubourdieu had planned, this detour opened Lissa to attack. Landing on the island with overwhelming force on 21 October, the French seized the shipping in the harbour but were unable to find the island's garrison, which had retreated to the mountains.