The Mischief Maker - E. Phillips Oppenheim

The Mischief-Maker is an early 20th-century political thriller and espionage novel written by British author E. Phillips Oppenheim, first published in 1913 (often cited around 1911–1913).
Pre-World War I Europe, moving between London, Paris, and broader cosmopolitan centers.
The story revolves around international diplomacy, secret agents, and political manipulation as tensions rise between Great Britain, France, and Germany