Sleipner entered service in 1936 and was handling neutrality incidents before Germany invaded Norway. In October 1939 it became involved with City of Flint, an American steamer brought into Norwegian waters by a German prize crew. Norwegian authorities warned that the ship would have to be handed over and the Germans interned if they remained. The Germans initially departed, but a later attempt farther south ended with another Norwegian vessel seizing the ship, interning the prize crew and freeing the Americans. These earlier disputes help explain why the later Altmark incident fitted an established pattern of German use of Norwegian waters.
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City of Flint tests Norwegian neutrality before the invasion
City of Flint's German prize crew forces Norway to decide how to enforce neutrality and protect the captured American crew.