'Istrien' (ii) was a German operation against Italian and Yugoslav partisan forces in north-eastern Italy and north-western Yugoslavia (25 September/20 October 1943).

The operation’s objective was the occupation of the Slovenian coast, Istrian peninsula, and part of the Kvarner coast as well as the area between Gorski Kotar and Ogulin in order to establish a land link with Generalfeldmarschall Maximilian Reichsfreiherr von Weichs’s Heeresgruppe 'F'.

After Italy’s armistice with the Allies in September 1943, there began an open uprising in the areas of the Slovene and Croat coasts, Istria, and Gorski Kotar, Notranjsko and Dolenjsko, which resulted in the disarmament of Italian units, the liberation of a significant area, and the establishment of several new partisan brigades as well as the 18^th, 30^th and 31^st 'Slovene' Divisions of the partisan army. The uprising also severed or severally disrupted the German lines of communication between Italy and the Sava valley.

The 194^th Grenadierregiment managed to secure the major ports of Trieste and Pola on 13 September, and Fiume and Susak two days later before the partisan forces could move in, but both Pula and Rijeka became were isolated by the partisan forces before a stalemate ensued. On 19 September the German high command ordered Generalfeldmarschall Erwin Rommel, commanding Heeresgruppe 'B', to suppress the uprising, eliminate the partisan forces in Gorski Kotar, eastern Slovenia, the Slovene and Croat coasts, and Istria, and secure the entire eastern coast of the Adriatic Sea in this region.

The task was entrusted to SS-Obergruppenführer und General der Waffen-SS Paul Hausser’s II SS Panzerkorps, which used more than 40,000 men, 95 tanks, 41 assault guns, 23 tank destroyers, six heavy self-propelled guns and some 300 half-track vehicles against a partisan force totalling some 15,000 persons. Their massive armoured superiority allowed the Germans to achieve their goals quickly against partisan forces possessing only a few captured Italian tankettes and anti-tank guns.

The operation was undertaken in three phases. In the first phase, between 25 and 30 September, the Germans took control of the Slovenian coast as the II SS Panzerkorps secured the area of Trieste, Tolmin, Idrija and Postojna as planned. In the second phase, between 2 and 7 October, the Germans took control of Istria and blocked all the partisans' avenues of escape in the direction of Gorski Kotar and the Kvarner coast. In this phase of 'Istrien' (ii), the German forces totalled 30,000 men, 106 tanks and 295 half-tracks vehicles. The German force comprised part of Generalleutnant Dr Franz Beyer’s 44^th Reichsgrenadierdivision Hoch- und Deutschmeister (132^nd Regiment reinforced by the 1^st SS Grenadierregiment and 21^st Panzerregiment) and Generalleutnant Wilhelm Raapke’s 71^st Division (194^th Regiment reinforced by the 901^st Regiment). These formations reached their start lines on 1 October, in the process destroying the partisan 'Trieste' Battalion, which happened to be in the area.

Lacking experienced officers, the partisan headquarters for Istria had already decided to withdraw most of its 12,000 soldiers (largely volunteers raised in the previous 10 days) to Gorski Kotar, but this escape route had been cut before the movement could be undertaken, and in the circumstances the Germans were able to achieve most of their objectives in only two days.

In the third phase of the operation, between 7 and 20 October, the Germans cleared the Istrian peninsula and Kvarner coast, and advanced through Gorski Kotar to link with the forces of Heeresgruppe 'B' at Ogulin. Because the operation’s second phase had been completed more swiftly then had been planned, preparations for the third phase were completed on 6 October. The partisan general staff for Croatia failed to react to this German concentration until it was too late, despatching regrouping orders only on 7 October, after the German attack had begun.

German reports claimed that the partisans had lost 4,096 dead and 6,850 captured, as well as losing 32 pieces of artillery and 2,182 infantry weapons. Later estimates suggest a partisan and civilian death toll of about 2,000, with another 500 to 600 sent to concentration camps. One of the primary goals of the operation had been the railway connecting Rijeka, Delnice, Ogulin and Karlovac, but this was found to be so severely damaged that it was considered unusable, and this contributed to a loss of German interest in the area.