Even dedicated history buffs may not know the exciting—and disturbing—story of German spy activity in the U.S. before our nation’s entry into the two World Wars.

Mein Spy gives a firsthand documentation of German espionage and sabotage in the U.S. in the lead-ups to both World War I and World War II. These activities, some only plotted, some carried out, defeated their own purpose. Instead of keeping America from intervening on behalf of the Allies, German disinformation and sabotage inflamed an American public opinion that up to that point had mostly been neutral.

Both of these books were first published before the U.S. entered the World Wars. The German Spy in America, published in 1917 (with a foreword by Theodore Roosevelt), details German attempts at sabotage and infiltration in the first two and a half years of World War I, including attempts to provoke Mexico to war against the United States, provisions for the invasion of Canada, and the sinking of the liner Lusitania in 1915, which cost the lives of 113 American citizens and shifted U.S. opinion decisively against Germany.

John J. Spivak’s Secret Armies: The New Technique of Nazi Warfare, was first published in February 1939, shortly before the outbreak of the Second World War. The book outlines “the activities of Nazi agents in the United States, Mexico, and Central America” in the late 1930s. “During the past five years,” Spivak writes, “I have observed some of them, watching the original, crudely organized and directed propaganda machine develop, grow and leave an influence far wider than most people seem to realize.” First aimed at domestic propaganda, these activities extended to probing U.S. military secrets and attempting to damage U.S. relations to its neighbors to the south.

The methods described here—letters, telegrams, personal conversations—may seem crude in the light of today’s advanced technologies. But these two compelling volumes show that if the technology has changed, the basic motives and principles remain the same—and cannot afford to be ignored.

Far from being a historical curiosity, Mein Spy provides invaluable and relevant insights that today’s informed citizens should know and understand.