October 27, 1942. In the Atlantic off the coast of northwest Africa, a wolf pack of German U-boats falls upon a slow convoy of merchant ships and begins tearing it apart, sinking ship after ship over several days. It was one more convoy massacre in the long Battle of the Atlantic, except for one extraordinary thing: by dying, this convoy may have saved one of the most important operations of the war. The convoy was SL-125, bringing merchant ships up from Sierra Leone toward Britain, the kind of slow, vulnerable convoy that the U-boats loved to hunt, the same sort of target as the SC-7 I told you about. A pack of submarines found it near the approaches to Gibraltar and set upon it in the running, multi-day battle that wolf-pack tactics had made so deadly, attacking at night on the surface, and over the days around the 27th they sank around a dozen of the convoy's ships, a brutal loss. But here is the thing that makes SL-125 unlike any other convoy battle. At that very moment, just over the horizon, an enormous Allied operation was under way. Hundreds of ships carrying tens of thousands of American and British troops were converging on the coast of northwest Africa for Operation Torch, the great Anglo-American invasion of French North Africa, the first large joint offensive of the war in the west, timed to come down on Rommel's rear as he reeled back from El Alamein. That invasion fleet was passing through the same waters, and if the U-boats had found it, the slaughter could have been catastrophic, troopships full of soldiers sent to the bottom before they ever reached the beaches. Instead, the U-boats found SL-125. The wolf pack that might have fallen on the invasion convoys was drawn to the slow merchant ships and spent itself attacking them, its submarines concentrated around SL-125 and out of position to intercept the Torch fleet, which slipped past and reached North Africa to make its landings successfully. The sacrifice of the merchant ships and their crews, unknowing, had in effect screened the invasion. There has long been a dark question hovering over SL-125: was it deliberate? Some have wondered whether the convoy was knowingly routed as a decoy, offered up as bait to pull the U-boats away from the invasion fleet, its merchant sailors sacrificed on purpose to protect the soldiers. The evidence is murky, and most historians think it was more likely a fortunate accident than a cold-blooded plan, that the convoy's suffering happened to draw the wolves at exactly the right moment. One British commander is said to have remarked, grimly, that it was the only time he was glad to see a convoy attacked. Either way, the result was the same. The men of SL-125 paid a heavy price in ships and lives, and their ordeal, deliberate or not, helped ensure that the Torch invasion fleet got through. It is one of the strange, poignant footnotes of the Atlantic war, a convoy whose destruction may have been its unintended gift, the merchant sailors who died off Africa so that an army could land, mostly unaware of what their sacrifice had bought.