The Zimmermann telegram incident was one of the key components that brought about the end of World War I. Zimmermann, the then-Foreign Minister of Germany, secretly contacted Mexico and tried to form an alliance to contain the United States and ultimately win the war. However, the telegraph was intercepted by the British intelligence agency, which resulted in a declaration of war against Germany by the previously neutral United States, thereby changing the course of world history. Using the most authentic records available, this book explores the intelligence activities and diplomatic strategies behind the First World War.