Athanasios Souliotis-Nikolaidis (1878-1945) was a Greek military officer, undercover agent, author, and politician who is not as well known in Greece today as he should be. Inasmuch as he is remembered at all today, Souliotis-Nikolaidis is associated with the much better-known Ion Dragoumis with whom he was connected with bonds of friendship and ideology. In this work the author examines the subject's role and contribution to Greece's irredentist activities of the early 20th century and answers some key questions. What were Souliotis-Nikolaidis's achievements as an undercover agent in Ottoman Macedonia? What was his behind-the-scenes role in the early elections of the Ottoman Empire following the Young Turk Revolt? What was his relationship with important individuals and organizations of the Greek Diaspora? What was his contribution to the unique idea about the future of Greeks and Turks in a unified federal state? In this work the author reveals that Souliotis-Nikolaidis, far from being a minor player in Greek irredentism was an important actor whose many contributions deserve recognition.