Dönitz made these ports the U-boats’ main operational base and set up his headquarters in a château outside Lorient, at the head of a long peninsula that sheltered what had formerly been a lazy fishing port. Standing on his sweeping terrace, he could see the largest of the concrete pens that Nazi engineering crews were building to shelter his boats. Fifteen thousand slave laborers were working on three colossal enclosures, each with a twenty-five-foot-thick reinforced-concrete roof. When completed in January 1943, these were among the most imposing defensive fortifications in the history of
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