Operation Royal Marine was a British plan developed in 1939-1940 to disrupt the German economy. The idea was to take advantage of the importance of the Rhine in the German transport system by dropping floating mines into the river close to Strasburg, where it formed the Franco-German border. These mines would then float downstream, sinking German shipping and damaging or destroying pontoon bridges across the Rhine. The idea had first been suggested by Winston Churchill in September 1939, but serious work did not begin until November, after the Germans had begun to drop magnetic mines in British waters.
The idea won support in the War Cabinet, and with General Gamelan. Admiral Penrose FitzGerald was appointed to head the project. At first the biggest problem was to develop the correct type of mine. The weapon would have to float just below the surface of the Rhine. It would need to have a “self-destruct” system to prevent it floating into neutral Dutch waters. Churchill also wanted it to explode if it got stuck for a long period of time.
These problems were soon overcome, and it was confidently expected that the operation could begin in March 1940. However, “Royal Marine” then ran into its most serious opponent – the French. Prime Minister Édouard Daladier was opposed to any offensive action on the Franco-German front, instead preferring to attack the German war economy by blocking iron ore supplies from Narvik and attacking Soviet oilfields from the Black Sea. On a mid-March visit to France to promote “Royal Marine” Churchill was even told that the “President of France had decreed that no aggressive action must be taken which would only draw reprisals on France”.
On 19 March Daladier’s government fell after the failure, and he was replaced by Paul Reynaud. He was more willing to support offensive operations. On 28 March the British and French war cabinets met in London, and Reynaud agreed to allow “Royal Marine” to begin on 4 April. In return the British agreed to lay mines outside the Norwegian port of Narvik, to prevent iron ore shipments reaching Germany (Operation Wilfred). The only flaw in the agreement was that Reynaud’s agreement was provisional on the approval of the French War Committee (Comité de Guerre). Daladier was now serving as Minister of Defense, and was still hostile to the operation. His argument now was the Germans would respond with air raids against the French aircraft industry, and he was able to convince the committee to block the operation, at least until July 1940.
Churchill made another visit to Paris on 4 April in an attempt to convince Daladier to change his mind, by which time Chamberlain had threatened to cancel the Narvik operation if Royal Marine was blocked again. This time it was Churchill who changed his mind, either because he was convinced by Daladier’s arguments (which seems unlikely), or because he realized that linking the two schemes would probably prevent either of them from taking place. Churchill had been an early supporter of mining Narvik. Whatever his motive, Churchill sent a message to Chamberlain asking for permission to remove the link between the two operations. By this point, the German troops earmarked for the invasion of Norway were already at sea.
Operation Wilfred, the mining of Narvik, eventually took place on 8 April. The naval forces involved became caught up in the German invasion of Norway, and the original operation was largely forgotten (once Germany had occupied Norway Operation Wilfred lost its purpose, as German shipping in Norwegian waters became a legitimate target of attack).
The French finally agreed to allow Operation Royal Marine to take place if the Germans attacked France or Belgium. That attack finally began on 10 May, and soon made projects such as Royal Marine seem entirely irrelevant. The operation itself did take place, and nearly 1,700 mines were dropped into the Rhine during its first week. Water traffic between Karlsruhe and Mainz almost came to a halt, and a number of German pontoon bridges over the river were badly damaged but whatever success the operation had was entirely overshadowed by the success of the German invasion of France and Belgium.