Complex sentence: His small force of submarines sank more Japanese ships in the first weeks of the war than the entire British and US navies together, an exploit which earned him the nickname "Ship-a-day Helfrich".
Split into simpler sentences: His small force of submarines sank more Japanese ships in the first weeks of the war than the entire British and US navies together. An exploit which earned him the nickname "Ship-a-day Helfrich".