He joined the Nazi Party in the interwar period, eventually rising to the rank of Korpsführer of the National Socialist Flyers Corps.

He was also responsible for the Dutch famine of 1944–1945 that resulted in the deaths of thousands of civilians after ordering an embargo on all food transports to the western Netherlands.

From 1915 to 1916 Christiansen went on numerous reconnaissance and bombing missions, helping to make his unit at Zeebrugge one of the most successful in the German Naval Air Service.

On 1 September 1917 he took command of Naval Air Station at Zeebrugge, and being promoted to Oberleutnant zur See, shot down a Porte FB2 Baby off Felixstowe the same day.

[6] Christiansen's distinguished career led him eventually to being called to a post in the Reichsluftfahrtministerium (RLM/Reich Aviation Ministry) from 1933 to 1937, and in 1936 he was promoted to Generalmajor.

[citation needed] From 29 May 1940 until 7 April 1945 Christiansen was Wehrmachtbefehlshaber in den Niederlanden (Supreme Commander of the Wehrmacht in the Netherlands), and until 26 June 1943 was concurrently still Korpsführer of the NSFK.

Christiansen also was responsible for the food embargo in winter 1944, causing famine in western Holland resulting in the death of 22000 civilian men, women and children.

On 2 October 1944 he had ordered a raid on the village of Putten in Gelderland, the Netherlands, in retaliation, after one of his officers, a Leutnant Sommers, was killed there by the Dutch resistance.

In compliance with this retributive sentiment, several members of the civilian population were shot, the village was burned, and 661 of the males of the town were deported to labor camps, the vast majority of whom never returned.

His release from imprisonment in 1951 was an occasion for his native town, Wyk auf Föhr, to renew Christiansen's honorary citizenship and reinstate a street name in his honor, previously changed by the British military administration in 1945.