forces invaded Poland. Following the 1939 aerial Battle of the Heligoland Bight, Royal Air Force (RAF) attacks shifted to the cover of darkness, initiating the Defence of the Reich campaign. By mid-1940, Generalmajor (Brigadier General) Josef Kammhuber had established a night air defense system dubbed the Kammhuber Line. It consisted of a series of control sectors equipped with radars and searchlights and an associated night fighter. Each sector named a Himmelbett (canopy bed) would direct the night fighter into visual range with target bombers. In 1941, the Luftwaffe started equipping night fighters with airborne radar such as the Lichtenstein radar. This airborne radar did not come into general use until early 1942. Night fighter career On 1 October 1941, Leutnant Greiner reported for duty with Nachtjagdgeschwader 1 (NJG 1—Night Fighter Wing 1) at Stade near Hamburg. Greiner was assigned to II. Gruppe (Group) under the command of Gruppenkommandeur (Group Commander) Walter Ehle. Greiner was transferred to 4 staffel in mid-January 1942. Along with 6 staffel it formed the nucleus of II./NJG 1, based at Leeuwarden, Netherlands. In February Greiner participated in Operation Donnerkeil (Thunderbolt), an air superiority mission to provide cover to the German Kriegsmarine (War Navy) and three major warships traversing the English Channel to bases in Germany. The battleships Scharnhorst and Gneisenau and the cruiser Prinz Eugen began the risky operation on 11 February. The naval component was called Operation Cerberus, and succeeded. On 20 April 1942 Greiner was promoted to Oberleutnant (First Lieutenant). In 1942 RAF Bomber Command, under the command of Air Officer Commanding, Arthur Harris, embarked on its long campaign of area bombing against German industrial cities and military targets. On the night of the 25/26 June 1942, 960 aircraft attacked Bremen, targeting the city, shipyards and the Focke-Wulf factory. 472 Vickers Wellington, 124 Handley-Page Halifax, 96 Avro Lancaster, 69 Short Stirling, 51 Bristol Blenheim, 50 Handley-Page Hampden, 50 Armstrong Whitworth Whitley, 24 Boston, 20 Avro Manchester and four de Havilland Mosquito bombers were used. A further 102 Lockheed Hudson and Wellington bombers of RAF Coastal Command were sent on the operation. Greiner was able to intercept a Wellington at daybreak, claiming it shot down south of Harlingen for his first victory at 06:00 with Bordfunker Rolf Kissing. On 6 October 1942, Greiner shot down his second victim west of Harlingen at 23:24. The bomber, a No. 103 Squadron RAF Halifax II, W1189, PM-E, crashed