The War At Sea, 1939

Germany had originally planned to restrict submarine attacks to warships and ships carrying war matériel in order to avoid provoking neutrals, as had happened during World War I. On the first day of the war, however, the U-30, acting contrary to orders, sank the British liner Athenia. Restrictions on submarine attacks were thereafter gradually reduced, until all enemy ships became potential targets.

On September 18, a German submarine sank the British carrier Courageous off the Scottish coast. The most spectacular victory for German U-boats came on October 14, when the U-47 penetrated the heavily guarded anchorage at Scapa Flow and sank the British battleship Royal Oak. The British had considered Scapa Flow's defenses to be impenetrable, and the U-47's feat caused them to abandon this base temporarily.

The Germans attempted to choke off British shipping by sowing magnetic mines, newly developed weapons that were triggered by the magnetic field of a ship's hull, across the entrances to British ports. To protect their ships from these mines, the British developed the degaussing belt, a cable through which a low-voltage current was passed. The cable was attached to the hull of a ship, neutralizing the ship's magnetic field, thus greatly reducing the danger from magnetic mines.

In order to skirt the disarmament provisions of the Treaty of Versailles and other international agreements, the Germans had built an intermediate class of ship that had the firepower of a battleship but the displacement and speed of a cruiser. These so-called pocket battleships inflicted serious damage on merchant shipping. In October, the German pocket battleship Deutschland destroyed several merchant ships and also seized the American freighter City of Flint, which was later released.

The pocket battleship Admiral Graf Spee conducted a particularly destructive raid in the southern Atlantic ocean, destroying nine merchant ships in the fall of 1939. The British cruisers Exeter and Ajax and the New Zealand cruiser Achilles damaged the Spee in a battle off the coast of Uruguay on December 13. The German ship took refuge in the neutral port of Montevideo, Uruguay, where, the Uruguayans insisted, it could remain for only 72 hours. Faced with certain destruction by the Allied ships waiting in international waters just outside the harbor, the captain of the Spee ordered it scuttled in the harbor on December 17.