This year is the centennial of the start of World War I, and there will no doubt be an outpouring of books assaying the conflict, particularly the carnage on the Western Front. Howard Blum's riveting and perturbing "Dark Invasion" examines a less devastating but still ruthless campaign: the Germans' ingenious espionage operations in the United States during the years when President Woodrow Wilson earnestly tried to maintain the nation's neutrality.

The Germans were convinced that they had the right and duty to wage...