Hooton, E. R. Franco's Pirates. Naval Aspects of the Spanish Civil War 1936-1939

The Spanish Civil War was won and lost upon the high seas. It was won because the Nationalists had an uninterrupted flow of men and materials while Republican sea lanes were attacked by Fascist warships, submarines, and aircraft - the pirates of the title. These attacks also involved dozens of foreign merchantmen and warships, including American, as well as hundreds of men, women, and boys. The worst affected was the British merchant marine, which dominated Spanish trade - some owners used rust buckets to maximise profits in a trade, which resulted in the loss of 66 British lives.
The naval element of the Spanish Civil War began with a rebellion followed by a mutiny and a massacre. Both the German and Italian navies became involved in the naval war, attacking Spanish ships and then British warships and merchantmen. A blockade in the north led to confrontations between the Royal Navy and Nationalist Navy, the mining of a British liner and tales of daring among determined British master mariners. Later in the war there were attacks by Italian surface warships, submarines, and aircraft against foreign shipping in the Mediterranean and Aegean leading to the British and French threatening pirate submarines.
This is a story of exploitation, heroism, chauvinism, piracy, international inaction, and espionage which has never been told. It includes details of such things as the first aerial campaign against shipping and the first operational use of sonar against a submarine.