Dark Invasion by Howard Blum

When a “neutral” United States becomes a trading partner for the Allies early in World War I, the Germans implement a secret plan to strike back. A team of saboteurs—including an expert on germ warfare, a Harvard professor, and a brilliant, debonair spymaster—devise a series of “mysterious accidents” using explosives and biological weapons, to bring down vital targets such as ships, factories, livestock, and even captains of industry like J. P. Morgan.

New York Police Inspector Tom Tunney, head of the department’s Bomb Squad, is assigned the difficult mission of stopping them. Assembling a team of loyal operatives, the cunning Irish cop hunts for the conspirators among a population of more than eight million Germans. But the deeper he finds himself in this labyrinth of deception, the more Tunney realizes that the enemy’s plan is far more complex and more dangerous than he suspected.

REVIEW:
This was the first I heard of Germany's secret war against the US before our entry into WWI- what a riveting story! The author used the personalities of those involved (on both sides) to pull the reader into the story. I was fascinated by how much work went into countering this early threat, and by how similar the circumstances and motivations are to those of today's secret warriors. Highly recommended!