[3] In 1914, he joined the Danish Army, not as a medical officer but as a recruit, and was promoted to Second Lieutenant in The Royal Life Guards a year later.

While there, he participated in an expedition to Central Asia (Mongolia and Tannu Uriankhai),[5] and in February 1918 was sent on a secret aid mission to Maria Feodorovna (Dagmar of Denmark) in Crimea, mother of the last Russian monarch, Emperor Nicholas II.

In 1922, he returned to Russia as leader of the Danish Red Cross delegation in Fridtjof Nansen's efforts to alleviate the Russian Famine of 1921-22.

[7] In 1922, Krebs organized and led an expedition to establish a farming, mining and fur trading settlement near Erdenebulgan in the Khövsgöl province of northern Mongolia, however the enterprise never prospered.

[9] In 1940 he was employed by the (then neutral) United States at their Berlin Embassy to monitor conditions of Allied Prisoners held in German camps.