Nikolaus Ritter is best known as the Chief of Air Intelligence in the Abwehr who led spyrings in the United Kingdom and the United States from 1936 to 1941.

The Abwehr was the German military-intelligence service for the Reichswehr and the Wehrmacht from 1920 to 1945. Although the 1919 Treaty of Versailles prohibited the Weimar Republic from establishing an intelligence organization of their own, they formed an espionage group in 1920 within the Ministry of Defence, calling it the Abwehr. The initial purpose of the Abwehr was defense against foreign espionage: an organizational role that later evolved considerably. Under General Kurt von Schleicher the individual military services' intelligence units were combined and, in 1929, centralized under Schleicher's Ministeramt within the Ministry of Defence, forming the foundation for the more commonly understood manifestation of the Abwehr.