In March 1943, German U-boats are on the attack – they sink 108 Allied vessels that month alone. Some Allied officials fear a German victory in the Atlantic is imminent. If the Allies lose the Atlantic, Britain loses its lifeline – and maybe even the war. But by May 1943, it will be the U-boats limping home in defeat. So how, in just two months, did the U-boats go from hunters to hunted?