Naval control zones

The Non-Intervention Committee established its system of naval patrols on 20 April 1937. By then the Germans and Italians on the one hand, and the Soviets on the other, had been providing military supplies to the rebels and the Republic respectively for many months, even though the were signatories to the Non-Intervention Agreement (NIA). 

The idea behind the system of naval control zones was that naval patrols would prevent foreign ships from carrying military supplies to the two sides. This map illustrates the design of the system. The coastal area in blue was patrolled by the German navy; the areas in Green by the Italian navy; the area in beige by the French navy; and the area in pink by the British navy. This map illustrates the design of the system. The coastal area in blue was patrolled by the German navy; the areas in Green by the Italian navy; the area in beige by the French navy; and the area in pink by the British navy.

The naval control zones system was set up so that the British and French would supervise the coasts controlled by the Francoists and the Germans and Italians the coasts controlled by the Republic. It got off to a bad start. Barely five weeks after the patrols began, a Nazi fleet bombarde the city of Almería, which was in the British zone, in broad daylight. And what followed was a farce, as there were many ways to evade the controls.

Ships carrying Spanish flags or those of countries that had not signed the NIA could be stopped or inspected. The Germans took advantage of this by sending their arms to Spain in ships that fraudulently flew the flag of Panama, a non-signatory nation, and Italian ships flew the Francoist flag when they were at sea. The Soviets sent their aid in Republican vessels.

The patrols could also not prevent attacks by Italian and German submarines on ships carrying arms and men to the Republic, or the nighttime attacks by Italian cruisers on Barcelona and Valencia. Nor could they control the coasts of Portugal, an ally of the rebels. They were also useless when the Republicans sent arms to France and then took them across the border, when it was open, by railroad.