General Walter Nicolai (August 1, 1873 – May 4, 1947) was the first senior IC[citation needed] (intelligence) officer in the Imperial German Army.

He came to run the German military intelligence service, Abteilung IIIb, and became an important pro-war propagandist in Germany during the First World War of 1914–1918.

After two years of service in early 1913, he was named the head of Abteilung IIIb, which helped to inform the Austro-Hungarian Army that Colonel and Evidenzbureau chief Alfred Redl was a Russian mole.

Mata Hari's mission was to reconnoiter the Third French Republic next offensive plans from Paris, travel through militarily interesting-areas of France and maintain contact with the Kriegsnachrichtenstelle (War Intelligence Office) West in Düsseldorf, whose director was Major Röpell, and the agent headquarters in the German embassy in Madrid, whose director was the military attaché Major Arnold Kalle.

[7] In January 1917, Major Kalle transmitted radio messages to Berlin that described the helpful activities of a German spy codenamed H-21 whose biography so closely matched Mata Hari's that it was patently obvious that she had to be the agent.

[9] In early 1917, Colonel Nicolai had grown very annoyed that Mata Hari had provided him with no intelligence worthy of the name, but only Paris gossip about the sex lives of French politicians and Generals.

For many years, she was invariably known as Mademoiselle Docteur or Fräulein Doktor, her actual name being revealed only in 1945 from German intelligence documents captured by the Allies after the Second World War.

[13] After the Second World War, Nicolai was arrested by the Soviet SMERSH under the personal order of Stalin,[14] deported from Germany and interrogated in Moscow.