US Navy
Naval Historical Center: The h.l. Hunley in Historical Context
An extensive history of the Confederate submarine, H.L.Hunley, the first submarine to sink a warship. Also information about two earlier prototypes.
Other
Destroyers Online: Destroyers, Destroyer Escorts, and Frigates
Extensive information about primary U.S. war ships used to protect convoys from U-Boats.
US Navy
Naval Historical Center: H. L. Hunley, Confederate Submarine
This site provides a short article on the history of the first submarine to engage and sink a warship. Many links to information about its history, preservation and excavation. Includes original US and Confederate Navy documents.
Curated OER
History Matters: The Zimmerman Telegram Bringing America Closer to War
This is a copy of the actual Zimmerman Telegram where Germany states that it "Intends to begin - unrestricted submarine warfare." A great primary source that takes students deeper into historical understanding.
Digital History
Digital History: The United States Wwi
This site explains how Germany resuming unrestricted submarine warfare contributed to the US entering WWI. Site is well-written, informative, and contains great details.