| Operation Northern March / Unternehmen Nordmark | |
|---|---|
| Part of the Phoney War of the Second World War | |
 Map of the North Sea | |
| Location | |
| Planned by | Marinegruppe West |
| Commanded by | Wilhelm Marschall Alfred Saalwächter |
| Objective | Attack British merchant shipping |
| Date | 18 February 1940 |
| Outcome | Inconclusive |
| Casualties | Nil |
Operation Northern Mark (Unternehmen Nordmark) was a sortie by a German flotilla of two battleships and a heavy cruiser against British merchant shipping between Norway and Shetland from 18 to 20 February 1940. The sortie was intended as a riposte to the Altmark incident, to create confusion to help German blockade-runners reach home and as a prelude to more ambitious operations in the Atlantic. The flotilla was spotted by the British early on, who held back a Norway-bound convoy.
Battleships of the Home Fleet were sent towards the North Sea to intercept the German ships. The German flotilla found only neutral ships and concluded that they had been found out; British submarine sightings of the flotilla came to nothing amidst stormy weather and confusion over conflicting reports from the submarines. The flotilla returned to base during the afternoon of 20 February; concurrent German U-boat patrols sank several ships, including a destroyer.