'Hydra' (ii) was a British air attack by Air Chief Marshal Sir Arthur Harris’s RAF Bomber Command on the German army’s 'secret weapon' development centre on Peenemünde island in the Baltic Sea just off the coast of Germany as the first element of the 'Crossbow' campaign (17/18 August 1943).

A combination of data from a number of intelligence sources regarding the A-4 (V-2) ballistic guided missile, such as the Oslo report (a letter written by the German mathematician and physicist Hans Ferdinand Mayer on 1/2 November 1939 during a business trip to Oslo, Norway, to detail several current and future German weapons systems, and posted anonymously to the British embassy in Oslo), interpretation of aerial reconnaissance photographs at RAF Medmenham, prisoner of war interrogations and Polish intelligence reports culminated in a critical meeting on 29 June 1943 of the British cabinet’s Defence Committee (Operations). The member of parliament Duncan Sandys (Prime Minister Winston Churchill’s son-in-law), appointed to the investigation in April, opened with an address about the rocket and showed photo-reconnaissance images of Peenemünde. Professor Frederick Lindemann followed and expressed weighty arguments regarding an 'elaborate cover plan' by the Germans and against the credibility of the reports and the existence of the suspected rocket. After Lindemann’s counter-argument, Churchill turned to Dr R. V. Jones, the physicist and scientific military intelligence analyst, who discredited each of Lindemann’s points. The committee agreed to recommend the termination of reconnaissance flights over Peenemünde lest these alert the Germans, and suggested that the site be attacked from the air. Churchill added that 'Peenemünde is…beyond the range of our radio navigation beams and…we must bomb by moonlight, although the German night fighters will be close at hand and it is too far to send our own. Nevertheless, we must attack it on the heaviest possible scale.'

At 10 Downing Street, the prime minister’s official London residence, on 15 July Churchill, Lindemann, Herbert Morrison (the home secretary) and the Chiefs-of-Staff examined the bombing plan, and the attack was ordered for implementation the earliest opportunity offered by lunar and meteorological conditions.

The nature of the raid was not revealed to the aircrews involved in 'Hydra' (ii): in their briefing the target was mentioned only as a development radar that 'promises to improve greatly the German night air defence organization'. In an effort to drive aircrews to do their utmost, their orders emphasised the importance of the raid with the words 'If the attack fails…it will be repeated the next night and on ensuing nights regardless, within practicable limits, of casualties.'

To ensure the maximum possible bombing accuracy, the crews were to drop their bombs on a full-moon night from an altitude of 8,000 ft (2440 m) rather than the normal altitude of 19,000 ft (5790 m). The task was made the more difficult be the fact that Peenemünde lay about 600 miles (965 km) from the most easterly British air base, covered a large area, and was protected by smoke screens. Bomber Command committed almost its entire strength to the raid, and undertook practice raids on areas similar to Peenemünde: margins of error of up to 1,000 yards (915 m) were initially recorded, but this was steadily reduced to 300 yards (275 m). The primary objective was to kill as many as possible of the scientific and technical personnel involved in the research and development of the V-weapons by bombing and destroying the workers' quarters. The secondary objectives were to render the research facility useless, and to 'destroy as much of the V-weapons, related work, and documentation as possible'.

The aircraft of Air Vice Marshal the Hon. R. A. Cochrane’s No. 5 Group had practised a timed run method for bombing, and were to use it on this occasion. The method involved the bomb-aimers of aircraft flying at a predetermined speed noting a distinctive point and then releasing at a set time, and therefore distance, from that point. The H2S nav/attack radar in use was at its best when it picked up contrasting areas of ground and open water, so the shoreline was chosen as the relevant distinctive point.

To divert German night-fighters from 'Hydra' (ii), a force of eight (or accordingly to another source 28) de Havilland Mosquito light bombers simultaneously flew the 'Whitebait' diversionary air raid on Berlin: it was hoped that this small force, by imitating the typical pathfinder marking of the target, would draw the main strength of the German night-fighter defences to the defence of the German capital. At 22.56 the first Mosquito of 'Whitebait' was over Berlin, and each of the light bombers dropped eight marker flares and a minimum bomb load.

Other activities related to 'Hydra' (ii) included two waves of Bristol Beaufighter long-range intruder aircraft of Nos 25, 141, 410, 418, and 605 Squadrons to attack the Luftwaffe airfields at Ardorf, Stade, Jagel, Westerland and Grove and any of their fighters which were taking-off or landing, and a mission by Handley Page Halifax bombers to drop supplies resistance groups in Denmark.

Throughout the 'Hydra' (ii) attack, the master bomber (Group Captain J. H. Searby flying in a bomber of No. 83 Squadron) circled above and around the target to call in new pathfinder markers and to direct crews onto the markers to be targeted.

The aircraft of Air Vice Marshal R. Harrison’s No. 3 Group and Air Vice Marshal C. R. Carr’s No. 4 Group targeted the V-2 scientists, and at 00.10 the first red spot fire was started. At 00.11, 16 blind illuminator marker aircraft began their marker runs with white parachute flares and long-burning red target indicators. Patches of stratocumulus clouds caused uncertain visibility in the full moon, however, and the H2S radar had not discerned Rügen island, to to the north-west, as had been planned, with the result that the red 'datum light' spot fires were placed inaccurately on the northern tip of Peenemünde Hook instead of burning as planned for 10 minutes on the northern edge of Rügen. The 2-mile (3.2-km) error later caused the misplacement of the early yellow target indicators on the Trassenheide camp. Searby noticed one subsequent yellow marker for the scientists' settlement 'very well placed' and ordered more yellow markers to be placed as close as possible to this first marker: four of six were accurate, as were three green back-up indicators.

At 00.27, the first wave of bombers withdrew after facing light Flak opposition, including a few pieces of heavy anti-aircraft artillery from a ship 1 mile (1.6 km) offshore and guns on the western side of the peninsula, under the control of Generaloberst Hubert Weise’s Luftwaffenbefehlshaber 'Mitte', but as yet no fighters. Some 33% of the 227 attacking aircraft were led astray by the false marking of the Trassenheide camp.

The attack by the bombers of Air Vice Marshal E. A. B. Rice’s No. 1 Group on the workshop area began by using 'aiming-point shifters' to mark the second aiming point via a bomb-sight offset back to the north-west along the bomb run from the first-wave marking. However, the correct solitary marker used for the first wave bombing was ignored, and the master bomber noticed the overshoot and notified the remaining backers-up, as well as the bombing force of 113 Lancaster aircraft.

The aircraft of Nos 3 and 4 Groups targeted the development works, and at 00.48 a backer-up accurately placed a green flare load in the heart of the development works for the third wave of bombers provided by Cochrane’s No. 5 Group and Air Vice Marshal G. E. Brookes’s No. 6 (Canadian) Group, and a few bomb loads caused serious laboratory and office damage.

As during the 'Bellicose' raid of June 1943 on the Zeppelin works at Friedrichshafen in southern Germany and La Spezia in north-western Italy, blind bombing after a timed run had been planned from Rügen in case smoke concealed the green target indicator. However, the Lancaster and Halifax bombers flew 20 or even 30 seconds past the timing point to the visible and inaccurate green markers from the six 'shifters' and three backers-up, their bombs landing some 2,000 to 3,000 yards (11830 to 2745 m) beyond the development works in the concentration camp for the foreign workers. At 00.55, as a result of timing errors, 35 straggler aircraft were still waiting to bomb.

The British official history states that the attack 'may well have caused a delay of two months', which is consistent with the German assessment by Dr Joseph Goebbels, the propaganda minister, of six to eight weeks. Although the raid was deemed 'not effective', in the US strategic bombing survey, Doctor Walter Thiel and Chief Engineer Walther were buried and killed in one of the location’s air raid trenches. Civilians at the Trassenheide camp were killed by bombs as they climbed the fence to flee, the camp’s gate being too distant. Although research and development continued almost immediately and test launches resumed on 6 October, plans for some V-2 facilities were changed after 'Hydra' (ii): for example, the nearly operational prototype production plant for the V-2 was moved to the Mittelwerk facility.

Bomber Command lost 6.7% of the 'Hydra' (ii) force, most of these losses happening in the course of the the final attack wave when the aircraft of General Josef Kammhuber’s night-fighter arm started to arrive. This was deemed an acceptable cost for what was deemed a successful attack on this important target on a moonlit night, during which some 560 aircraft dropped nearly 1,800 tons of ordnance, 85% of this tonnage being HE bombs. Most of the casualties were suffered by the aircraft of the last wave, which attacked after German night-fighters had arrived in force. The groups involved in this last phase were No. 5 Group, which lost 17 of the 109 aircraft (14.5%) which it despatched and No. 6 (Canadian) Group, which lost 12 out of 57 aircraft (19.7%).

After the Luftwaffe had realised that 'Whitebait' was a deception, its counterattack included about 30 Focke-Wulf Fw 190 'Wilde Sau' night-fighters which shot down 29 of the 40 bombers lost during 'Hydra' (ii). The counterattack also included the first operational use of the new schräge Musik weapon, which comprised two 20-mm cannon installed in the central part of a night-fighter’s fuselage to fire obliquely upward and forward into the belly of a bomber as the night-fighter formatted below and behind it: two Messerschmitt Bf 110 night-fighters with this installation found the bomber stream flying home from Peenemünde and are believed to have shot down six of the bombers lost on the raid.

After the success of the 'Whitebait' deception, the Luftwaffe’s chief-of-staff, Generaloberst Hans Jeschonnek, shot and killed himself on 19 August: Jeschonnek had ordered Berlin’s air defences to fire upon 200 German fighters, in the belief that they were British aircraft, which had gathered near Berlin after the 'Whitebait' deception worked.

Generalmajor Dr Walter Dornberger, head of the Peenemünde group, later stated that 'Hydra' (ii) missed damaging two vitally important installations (the wind tunnel and the building housing the 'measuring unit'), so minimising delay to the resumption of German research and production.

After 'Hydra' (ii), Peenemünde fabricated visual 'evidence' of bomb damage by creating craters in the sand (especially in the area near the wind tunnel), blowing-up lightly damaged and minor buildings and, according to one Peenemünde scientist, painting black and white lines on the roofs of buildings to simulate charred beams.

The weapons used in the 'Hydra' (ii) attack included bombs with delay-action timers set for up to three days, so along with bombs that had not detonated because of the sandy soil, explosions well after the attack were not uncommon and hampered German salvage efforts.