Operation Tanne Ost ("Fir East") was a German operation during World War II to capture the island Suursaari (; ) in the Gulf of Finland before it could fall into Soviet hands. Suursaari was especially important because it worked as a lock in the Finnish Gulf guarding the minefields keeping the Soviet Baltic Fleet in Kronstadt.
Background
Finnish preparations to separately conclude the war against the Soviet Union prompted Germans to prepare for such an eventuality. Furthermore, as the situation became critical in mid-June 1944 during the initial successes of the Soviet Vyborg-Petrozavodsk Operation Germans placed the forces prepared for the operation into full readiness. German plans for both of the Tanne operations were planned alongside the Operation Birke, the German 20th Mountain Army's evacuation and on 5 July 1944, the Tanne operations were assigned solely to the Kriegsmarine.
On 2 September 1944, Finland officially informed Germany of the coming cease-fire between Finland and the Soviet Union, which led to Hitler's orders to confiscate all Finnish shipping and to prepare for Operation Tanne Ost. At midnight, when a pair of Finnish VMV-class patrol boats were preparing to leave to transport a stranded German radio unit, a German minesweeper arrived at the docks on the eastern side of the island and demanded that the island surrender. When the demand was refused the Germans started to land troops which led to Finnish forces opening fire at 00:55 on 15 September 1944.
Consequences
This operation marked the beginning of hostilities between German and Finnish troops, known as the Lapland War. The Finns benefitted from the operation since it showed the Soviets that the Finns were prepared to use force against the Germans. It further damaged the German efforts in northern Finland since Finns ordered all shipping, including that leased to Germans, immediately to sail for Finnish or Swedish ports.