Complex sentence: Smith's troops had arrived by river on November 30 and their transports had been escorted by a powerful fleet of tinclad and ironclad gunboats; thus, the river barrier was well-defended.
Split into simpler sentences: Smith's troops had arrived by river on November 30, and their transports had been escorted by a powerful fleet of tinclad and ironclad gunboats. Thus, the river barrier was well-defended.