Operation Chastity was a World War II plan by the Allies to seize Quiberon Bay, France, enabling the construction of an artificial harbor to support Allied operations in Northern France in 1944. The artificial harbor was not developed, as the US VIII Corps failed to capture German-held areas that threatened the port. By the end of August 1944, Allied forces had captured all of Brittany except for the critical areas, preventing the further development of the operation. Following the capture of Antwerp and its port facilities in early September 1944, Operation Chastity was officially cancelled on 7 September. The non-completion and eventual cancellation of Operation Chastity exacerbated strains on the Allied logistical system, may have prevented an Allied victory over Germany in 1944, and has been described as "the critical error of World War II", although other historians believe that priority was rightly given to the pursuit of the fleeing German forces.
Background
One of the primary concerns when the Allies were planning Operation Overlord was the acquisition of deep-water ports. This was because the vital factor in its success was logistics, particularly American logistics, as the Twelfth United States Army Group would be operating further from the English Channel coast than the British 21st Army Group. The Overlord plan envisaged the Allies establishing a secure lodgement west of the Seine and north of the Loire. From this base, the Allies would advance to Paris and then Germany, once sufficient forces and supplies were available. The Overlord plan assumed that in the first weeks after D-Day, the Allied forces would be supported over the invasion beaches, and through two Mulberry harbours that would be assembled - "A" at Omaha Beach and "B" at Gold Beach. However, in the longer term and in order to handle the large quantities of reinforcements and supplies needed for the campaign, quayside discharge of Liberty ships would be necessary. Therefore, one of the first objectives of the Overlord plan was to seize the port of Cherbourg. After this foothold was secured, the most important single strategic objective would be the capture and development of major ports. Seven ports in Normandy were expected to be captured and opened in the first four weeks after D-Day: Isigny-sur-Mer, Cherbourg, Grandcamp-Maisy, Saint-Vaast-sur-Seulles, Barfleur, Granville, Manche and Saint-Malo. Except for Cherbourg, all were small and tidal, making Cherbourg the only major port supplying the Allied force. However, even Cherbourg was planned to develop a capacity of no more than 8 or 9,000 tons per day, while the minor ports were intended only as a stopgap. The inadequate port capacity in Normandy meant that the Brittany ports would have to play the key logistical role. In fact, the overall success of Operation Overlord was predicated on organizing Brittany as the principal supply base for US forces and the importance of Brittany in the Overlord plan "can hardly be exaggerated". Therefore, the Overlord plan called for Brittany to be isolated and its ports captured. Only after the ports were seized would operations towards Paris and Germany begin. However, all the ports were expected to have facilities destroyed, harbors blocked and approaches mined by the Germans, requiring extensive repairs before they could be used efficiently. Therefore, the planners concluded that the logistical capacity of the captured French ports would not be adequate to support the Allied liberation of France, and a new port had to be built somewhere in Normandy or Brittany.
Plan
The solution to the Allies logistical problem was devised in April 1944 and given the name Operation Chastity. It involved constructing a brand-new, deep-water port at Quiberon Bay. Quiberon Bay, a large estuary with four small ports, between Lorient and Saint-Nazaire on the southwest coast of Brittany, was sheltered by the Quiberon peninsula and a line of small islands, and the larger Belle Île. The Auray river had scoured a 3,000-yard (2,700 m)-long 80-foot (24 m)-deep pool between 30-yard (27 m) and 300-yard (270 m) wide with nearly vertical sides near the port of Locmariaquer. Operation Chastity planned to construct floating piers in the pool, allowing large ships to tie up alongside, with causeways to carry cargo and troops to the shore. There would be room for offloading five ships simultaneously, providing a capability of 2,500 tons of supplies per day directly onto vehicles. A further 7,500 tons per day could be offloaded using lighters carrying supplies directly to the shore from thirty further ships moored in the pool. The key advantage of Operation Chastity would be the ability to off-load Liberty ships sailing directly from the United States. A further advantage would be access to the relatively undamaged rail network outside the Normandy region, once a spur line and marshalling yard were constructed. The beaches of Quiberon Bay would also allow the unloading of LSTs at low tide. Other attractive features of Operation Chastity were the sheltered anchorage in Quiberon Bay, that the port required only a fraction of the labor and materials committed to the Mulberry ports, and that it used standard components and available equipment. However, the approaches to Quiberon Bay were covered by German coastal artillery at Lorient and on Belle Île and in the view of the planners, unless Brest, Lorient and Belle Île were captured, shipping cargo via Quiberon Bay would be, "impossible due to naval interference." Quiberon Bay was scheduled to be captured by D+40 with Brest and Lorient to fall by D+50. Planners thought that by D+60, British forces would be supported by the Mulberry ports, while American forces would be supported from Cherbourg, Saint-Malo, and Quiberon Bay.
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