'Tanne Ost' was a German unsuccessful attempt to seize the island of Suursaari (Hochland in German) from their erstwhile co-belligerents, the Finns (14/15 September 1944).

The operation, as originally planned, had envisaged that both Suursaari in the Gulf of Finland and the Åland islands group in the mouth of the Gulf of Bothnia should be taken simultaneously, in 'Tanne Ost' and 'Tanne West' respectively, to prevent the Finns handing them to the Soviets at the time of the armistice which ended the 'Jatkosota' continuation war. Suursaari was especially important as it effectively controlled the eastern end of the Gulf of Finland, and the installation of a German force on the island, guarding the local minefields, could effectively block the Vitse-Admiral Vladimir F. Tributs’s Baltic Fleet at Kronshtadt near Leningrad.

In the event, political considerations coincided with the logistical factor of the the German possession of shipping sufficient for only one of the operations, and on 11 September Adolf Hitler chose the landing on Suursaari, for implementation within one weel, as this area was closer to the main Soviet naval base at Kronshtadt than the Åland islands group.

Finland was currently negotiating with the USSR for an armistice, and the Soviets had agreed to negotiate if all foreign troops had left Finland by 15 September. Hitler was incensed at this defection, and ordered the operation in the belief that the Finns would not oppose the German landing. A first assault wave of 1,400 German army and navy troops was loaded at Tallinn on 15 September and prepared for the short passage to Suursaari. Under the command of Kommodore Karl-Conrad Mecke, the German force, of littler more than battalion strength, was carried in the vessels of Korvettenkapitän Dr Emil Kieffer’s 3^rd Minensuch-Flottille, Kapitänleutnant Heinz Vogeler’s 25^th Minensuch-Flottille, Kapitänleutnant Oswald Reinhardt’s 13^th Landungs-Flottille, Korvettenkapitän Hans Möhring’s 21^st Landungs-Flottille, Korvettenkapitän Erich Brauneis’s 24^th Landungs-Flottille, Korvettenkapitän Dr Theodor Sonnemann’s 7^th Artillerieträger-Flottille, Kapitänleutnant Carl Hoff’s 1^st Räumboots-Flottille, and Kapitänleutnant Hermann Holzapfel’s 5^th Schnellboots-Flottille.

Before committing his assault force, Mecke attempted to reach a negotiated settlement with Everstiluutnantti Martin Miettinen, the commander of the Finnish 12^th Coastal Artillery Regiment on Suursaari, who had earlier been reported as saying that he would never fire on German forces and might be prepared to abandon the island without resistance. The negotiations were a complete failure and the assault was therefore sent in.

As the German ships approached Suursaari, the Finnish forces on the island opened fire. Most of the troops made it to the beach, but there the Germans' difficulties continued, in part because the strength of the Finnish defence, totalling some 1,610 men, was somewhat greater than the Germans had estimated. After the break of day, Soviet aircraft bombed the German ships, and later bombed both the German and Finnish positions in the island.

A second 1,300-man wave of effectively untrained naval personnel was withdrawn before it could land, and Mecke opted to withdraw because he had failed to make radio contact with the troops ashore. The German forces which had landed were therefore abandoned, and were compelled to surrender on 15 September: in addition to 153 men killed, the Germans thus surrendered 1,231 men, including 175 wounded.

On 15 September the minesweeper R 29 was torpedoed and sunk off Suursaari by a Finnish motor torpedo boat, Taisto 5. R 76 was run aground after being hit by Finnish gunfire, but was salvaged. In overall terms, therefore, 'Tanne Ost' was a complete German failure, the naval losses being three landing craft (F 173, F 175 and F 177), three patrol boats, one minesweeper and one tug. The Finns lost only 36 men killed, 67 wounded and eight take prisoner, as well as two patrol boats.

During 'Tanne Ost', the Soviet air force undertook three attacks against the German positions and ships involved in the operation, and the attacks also caused some Finnish casualties. It was the threat of Soviet air intervention, as confirmed by events, which had led the Germans not to deploy the heavy cruiser Prinz Eugen for support of the undertaking.

This operation marked the beginning of the so-called 'Lapland War' hostilities between German and Finnish troops, but before this the Germans had completed their withdrawal from northern Finland without incident. The effect of the operation was a further strengthening of the Finnish determination to secure an armistice with the USSR, a preliminary agreement being signed on 19 September.