Having served in the Norwegian Army prior to emigrating to the United Kingdom, Krefting returned to active service following the German invasion of Norway in April 1940. Krefting participated in the Norwegian campaign as a liaison officer, attached to the British 148th Infantry Brigade when it deployed to fight the German forces that had landed in Norway. At the time of the campaign, Krefting held the rank of captain. After following the 148th Infantry Brigade on its deployment to the Moelv area in Eastern Norway, Krefting was ordered by the Norwegian High Command to return to the United Kingdom to join the Norwegian Military Mission there. The Norwegian Military Mission had as its task to mobilize Norwegians in the United Kingdom for service with the Norwegian forces fighting in Northern Norway. Before any units of expatriate and emigrant Norwegians could be formed and trained, the campaign in Norway ended in June 1940, and the Norwegian government evacuated to the United Kingdom. After the end of the fighting in Norway, Krefting worked on personnel issues for the exiled armed forces and the Norwegian merchant navy.