The 'Moonsund Defensive Operation' was the Soviet unsuccessful undertaking to hold the four islands of the Moonzund archipelago (Saaremaa, Hiiumaa, Muhu and Vormsi, or Ösel, Dagö, Moon and Worms in German) in the mouth of the Gulf of Riga in Soviet-occupied Estonia against the German amphibious assaults of 'Beowulf' (6 September/22 October 1941).

Even before the start of the German 'Barbarossa' invasion of the USSR on 22 June, German aircraft had been making reconnaissance flights over the western part of the USSR, and on 22 June switched to offensive operations, which included a major road on Moscow. Over this same period, German military analysts had come to believe that the USSR, which had become one of the major exponents of the theory of strategic bombing during the early and middle parts of the 1930s, would nit undertake retaliatory bombing raids as the Soviets seemed to have scaled back their interest in the later part of the same decade.

Nonetheless, the Soviet high command decided to put the lie to the German belief in the invulnerability of the German capital to bombing and to demonstrate the capabilities of its air forces by raiding Berlin in the 'Berlin Bombing Offensive Operation'. The only place not yet occupied by the Germans at that time, and from which Berlin was accessible by air, was the Moonzund archipelago. In the strictest secrecy, therefore, on 3 August a convoy of small ships from Kronstadt arrived in Saaremaa island’s Kuressaare (Kingisepp) bay with a cargo of bombs, ammunition and logistical support. The 1,470-yard (1,300-m) earth runway of Cahul airfield was then expanded by the Soviet garrison and support forces to a length sufficient for the operation of Soviet heavy bombers. Between 7 August and 5 September Soviet Ilyushin DB-3 (il-4) twin-engined and Petlyakov TB-7 (Pe-8) four-engined bombers of the Aviatsiya Dal’nego Deystviya (long-range aviation) and the air force of the Baltic Fleet flew 10 raids on Berlin and other targets, dropping 311 bombs with a total weight of 79,475 lb (36050 kg) including 46,300 lb (21000 kg) on Berlin. The Soviet losses were 17 or possibly aircraft in this first, albeit largely symbolic, Soviet air victory against Germany.

The 'Berlin Bombing Offensive Operation' greatly concerned the Germans inasmuch as it might be only the precursor of an altogether weightier and more successful Soviet air campaign. Soon after the first raids, Adolf Hitler demanded 'joint efforts of the formations of the ground forces, aviation and navy to liquidate the naval and air bases on the islands of Dagö and Ösel, and, first of all, the airfields from which the raids on Berlin are being carried out'. After the Soviet loss of Tallinn on the northern coast of Soviet-occupied Estonia and the relocation of Vitse Admiral Vladimir F. Tributs’s Baltic Fleet to Kronstadt at the eastern end of the Gulf of Finland between 27 and 29 August, the delivery of supplies to the the air group on the Moonzund islands became impossible, and the raids therefore ended.

After the Soviets' 27 June abandonment of the their naval bases at Liepāja and Ventspils (Libau and Windau in German) on Latvia’s Baltic Sea coast, also abandonment of the Latvian port city of Riga on 1 July, the ships of the Baltic Fleet departed for the ports of the Moonzund islands and Tallinn. Based on the Moonzund islands, small air units of the Baltic Fleet continued to harass German communications in the Gulf of Riga.

At the end of August, when the Germans broke through to Tallinn, the Soviet supreme command decided to evacuate Tallinn, the garrison of the Moonzund islands and the naval base at Hanko in south-western Finland. But this decision was carried out only in part, largely by the Soviet troops in Tallinn. On 28 August, Soviet troops left Tallinn and suffered heavy losses in men, warships and transport ships during their passage from Tallinn toward Leningrad. The evacuation of the Moonzund islands' garrison was not effected as, as a result of Soviet warship and transport Vessels in the Tallinn undertaking, the USSR no longer possessed sufficient capability.

At the beginning of August, the Baltic Fleet’s strength included a division of low-speed minesweepers and mineclearing small craft, as well as a number of slightly larger patrol vessels: these assets included 10 tugs (21, 22, 23, 25, 26, 29, 33, 34, 31 and 83), the survey vessel Hydrograph, 13 KM-type small minesweepers, and the minesweepers Sneg, Oblako and Virsaytis. The last three departed for Tallinn at the end of August and then became part of the evacuation of the Baltic Fleet to Kronstadt.

Under the command of General Major Aleksei B. Eliseyev, the 23,663-man garrison left on the Moonzund islands comprised the 6,500 men of the 8^th Army’s 3^rd Separate Brigade, separate units in the form of one battalion of sailors, two battalions of the 16^th Division’s 249^th Regiment, one Estonian battalion, four separate infantry companies and border detachment units. These had 142 coastal, field and anti-aircraft guns, 60 mortars and 795 machine guns. The coastal artillery element comprised 16 batteries including three four-gun turrets with 180-mm (7.09-in) guns, one 152-mm (6-in) gun, eight 130-mm (5.12-in) guns and three 100-mm (3.94-in) guns. Also on the islands were one platoon of four or five light smokelaying tanks, two tankettes and five or six armoured tractors; 12 fighter; four separate engineering battalions; and a naval component with six motor torpedo boats, 17 minesweepers, six motor boats, one tug and one transport vessel.

Against this Soviet strength Generalleutnant Georg von Küchler’s 18^th Army committed more than 50,000 men of Generalleutnant Siegfried Hänicke’s 61^st Division, Generalleutnant Richard Baltzer’s 217^th Division, one Finnish infantry battalion, field artillery, two engineer regiments, one pontoon regiment and as many as 60 aircraft. In support the German land forces had as many as 350 miscellaneous landing craft including the 128^th Pionierlandungsbataillon with 26 Siebel-type ferries), 182 assault boats and 140 other craft. Heavier support was provided by the light cruisers Leipzig, Köln and Emden, one destroyer flotilla, two torpedo boat flotillas, two minesweeper flotillas, one submarine hunter flotilla, and seven floating batteries of light and heavy artillery.

The Germans also planned an executed a number of diversionary operations including 'Südwind' with 50 vessels (transports, destroyers and patrol boats); 'Westwind' with the 2^nd Zerstörer-Flottille, 2^nd Torpedoboots-Flottille, 3^rd Torpedoboots-Flottille, three transport vessels, transports, three patrol boats and three minesweeping boats; and 'Nordwind' with the coastal defence battleships Ilmarinen and Väinämöinen, four VMV-type patrol boats, the icebreakers Yayakarnu and Tarma and one transport vessel of the Finnish navy, and the minelayer Brummer, five patrol boats and the tugs Mokaun and Typhon of the German navy.

On 6 September, the Germans began 'Beowulf' from ports on the Estonian coast. On the same day, Soviet coastal batteries on Osmussaar (Odinsholm in German), an Estonian island in the mouth of the Gulf of Finland some 4.67 miles (7.5 km) off the mainland, repulsed a German attempt to land.

On 8 September, the Germans landed on Vormsi and had completed its seizure after three days of fighting.

On 14 September, the Germans landed on Muhu, whose garrison held out for four days before its last defenders surrendered.

On 17 September, fighting broke out on Saaremaa Unable to delay the German offensive across the island’s full width, the Soviets withdrew to the island’s narrow-necked southern peninsula, which was well suited for defence and prepared fresh defensive positions. Stubborn battles were fought on this peninsula for some two weeks. On the night of 3 October, a small portion of the Sõrve peninsula’s garrison was evacuated to Hiiumaa. On 5 October, the Germans captured the demolished Coastal Battery No. 315, whose wounded commander was taken prisoner.

On 12 October, the Germans landed on Hiiumaa, where there was also fierce fighting before the Soviets started to concede defeat. Accordingly, the Baltic Fleet decided to evacuate the remnants of the garrison, of which 570 men were removed between 14 and 22 October by a motor boat ferry operation to the Soviet base on the Hanko peninsula in south-western Finland.

The considerable majority of the Soviet troops entrusted with the defence of the islands were killed or taken prisoner. The losses of the Soviet forces under the command of the Baltic fleet were 2,760 men killed or missing and 12,875 men taken prisoner, while as many as 700 men were evacuated and about 200 men managed to escape to Sweden, where they were interned. The Northern Fortified Sector lost about 230 men killed or missing, and 3,800 men taken prisoner. while the rest were evacuated.

The German losses are known only in part: between 9 September and 5 October the casualties totalled 472 men killed, 1,490 men wounded, and 92 men missing. Some 63 German aircraft were shot down or damaged.

In overall terms, the stubborn Soviet defence of the islands slowed the planned activities of the German fleet in the Gulf of Riga and the Gulf of Finland, diverted significant German forces to an area that was by this time nothing but a strategic backwater and could have been left to degrade into impotence, and weakened the German ground strength for the advance on Leningrad.