Complex sentence: The first French submarine to enter the Sea of Marmara was but it was forced to turn back and on 30 October, when attempting to pass back through the straits, ran aground beneath a fort and was captured intact.
Split into simpler sentences: The first French submarine to enter the Sea of Marmara was. However, it was forced to turn back and, on 30 October, when attempting to pass back through the straits, ran aground beneath a fort and was captured intact.