As a German plane buzzed overhead, nurse Helen Dore Boylston dropped face down in the mud. Boylston, an American nurse, serving at a British Army base hospital near the Western Front in 1918, had been running between wards of wounded patients that night, trying to calm their nerves during the air raid. Now, all she could do was brace herself for the hissing bomb that hurtled toward her. She covered her eyes and ears against the deafening roar and "blood-red flare." About a half-hour later,...