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Isolation of Spain would involve a blockade of the Strait of Gibraltar. To protect a French blockade of the Spanish Mediterranean coast France could not afford for long to leave Italians in the Balearic Islands. Italian Sardinia with its naval bases at Cagliari and Maddalena is the one bad break in an Allied north-south front that extends from Flanders to Bizerte, in French Tunis. Its capture by the French—a tough proposition in face of current military prejudice against combined land and water expeditions—would shake Italian control of the Central Mediterranean.
Contrary to popular impression, the French do not desperately need their Algiers-Marseille route to bring North African recruits into France and French reinforcements to Tunis. They have an alternative route, extending by rail along the North African coast from Bizerte to Casablanca on the Atlantic Ocean, thence to Bordeaux by transport under cover of the French and British Atlantic fleets. The French have built a big naval base at Bizerte; and they have an excellent harbor for submarines at Mers-el-Kebir, halfway between Algiers and Gibraltar. Mers-el-Kebir can be used to blockade the Strait of Gibraltar from the Mediterranean side, for the French have constructed oil and gas tanks and even submarine slipways under the protecting coastal rock.
The Po Target. Italy herself is extremely vulnerable—a fact which might cause Mussolini to sit out the next World War. Flying from bases in southern France and their advanced refueling fields in Corsica, the French could with the greatest of ease bomb Rome, La Spezia, Genoa, Naples and the railway that skirts the Italian coast. Airplane attacks from the sea come without warning.
The Po Valley, seat of Italian industry, is not only a sweet target for air raids but a natural magnet for enemy armies. Time without end the Po Valley has been invaded from France and Switzerland; but never since Gaul was civilized has there been a successful invasion of France or Switzerland from the Po. Napoleon won his great early victories in the Po Valley: at Lodi and Rivoli in 1796 and 1797, and at Marengo in 1800. And long before Napoleon's time, Hannibal crossed the Alps into Italy with elephants and a full baggage train, probably via the Little St. Bernard or the Mt. Genevre pass (the northernmost and the middle of the passes west of Turin).
The Po region is very open to attack if the French can once accomplish the none too easy job of forcing the passes, all of which converge toward Turin. Army corps starting from widely separated depots in southeastern France would meet when they came down into flat fighting country around the important Italian industrial centres, whereas Italian divisions invading France would find themselves scattered in rough mountain country a long distance from the Rhone Valley, from Marseille and from Lyon.
Since Italy and Germany are friendly, the Brenner Pass which leads from the Po Valley to Austria and Bavaria would naturally figure in the next war, chiefly for communication and supply.