Complex sentence: During the American Civil War, the Union-built and French-designed submarine "Alligator" was the first U.S. Navy submarine and the first to feature compressed air (for air supply) and an air filtration system.
Split into simpler sentences: During the American Civil War, the Union was the first to field a submarine. The French-designed "Alligator" was the first U.S. Navy sub and the first to feature compressed air (for air supply) and an air filtration system.