In June 1941 the unit's Gruppen participated in Operation Barbarossa and spent the next years on the Eastern front until 1944, until it was withdrawn to assist the evacuation of the German-occupied Greece and Yugoslavia.

At this time, all three combat groups remained operational but were converted to fighter units for Defence of the Reich duties.

KG 27 had to fly from bases in Hannover (450 mi; 725 km) for the most part and returned to forward landing grounds in Pomerania.

Colonel Stefan Pawlikowski's Pursuit Brigade offered fierce resistance but lost 17 percent of its strength as Messerschmitt Bf 110 destroyer groups protected the bombers.

[8] On the third day the OKL decided to end the counter-air campaign and strike at Polish aircraft industry only for Hermann Göring to change his mind.

Third group was assigned back to Albert Kesselring's air fleet and bombed rail targets around Warsaw.

[20] III./KG 27 bombed targets in the Lille, Antwerp, Brussels and Namur area in the same time frame and also attacked Wavre on 16 May.

[2] At this time, one recorded action on 14 May involved I and III./KG 27, which was supported by low-level Henschel Hs 123s from II(S)./LG 2 and escorted by II./JG 2.

[11] By 14 June Alfred Keller commanding IV Fliegerkorps sent out armed reconnaissance aircraft to find targets in an increasingly fluid situation.

[30] The following day unprotected bombers from the third group bombed Southampton where the main rail line was blocked.

Apart from Liverpool, which was hit by 12 aircraft, Sheffield, Derby, Hull, Nottingham and Leicester were raided by a small number of bombers and there were 112 British casualties.

[36] I./KG 27 flew mainly maritime patrol operations between Brittany and Ireland from mid-August to early September.

[37] The second group continued with night attacks on Liverpool, Birkenhead, London, Birmingham and Coventry until 29 August.

On 30 August it lost one bomber in a daylight raid over the English coast to 601 Squadron and another was brought damaged by ground-fire over the West Midlands.

All three groups were involved in Operation Mondscheinsonate (Moonlight Sonata), which was the code word for the attack on Coventry on 14 November 1940.

[44] Other towns were also bombed; Portland, Exmouth, Bognor Regis, Bournemouth, Cheltenham, Poole, Weymouth, Portishead, Avonmouth, Gravesend, Bridgeport, Swansea, Filton, Yate, "Stoke" (which one is unknown), Pembroke, Braunton, St Athan, Clydeside, Nottingham and Derby.

The following day III./KG 27 suffered five bombers and damaged and then on 10 July lost its commanding officer Fritz Reinhard killed in action.

Major General Yermachenkov's VVS ChF frustrated the German air and land advance.

The Axis air forces were unable to prevent the evacuation of 350,000 soldiers and civilians and 200,000 of arms and material during the Siege of Odessa, although the city did fall.

From 4 to 12 October, the group attacked troop columns, trains, airfields, and harbours in the Melitopol, Berdyansk, Rostov area.

[53] On 12 November nine He 111s suffered a loss when a MiG 3 piloted by Lieutenant Yakov Ivanov brought down Wilhelm Hofmann's bomber by ramming as the bombed the port of Gelendzhik.

It flew against targets in the Izyum area and the ports of Novorossisk (7 March), Sevastopol, Feodosiya mainly.

[57] KG 27 supported the German 6th Army attack on Volchansk, to acquire a staging area for Operation Blue.

KG 27 also returned to the Voronezh area bombing bridgeheads on the east bank of the Don; in one day I./KG 27 flew 14 missions against targets heavily defended by anti-aircraft artillery.

[60] After the Soviet Operation Uranus surrounded Axis forces in the city, the remaining groups took part in desperate counterattacks.

With other units, KG 27 inflicted heavy losses to Soviet infantry and horses on 25 November in action along the Chir river.

KG 27 assisted with the failed airlift but third group was sent to Hannover to rebuild and rest from December 1942 until 14 January 1943.

They also flew anti-tank missions; on 11 April crews claimed 44 Soviet tanks in operations over the Perekop Isthmus.

[67] I./KG 27 flew night combat missions against rail and airfields during the Soviet summer offensive Operation Bagration.

By 30 June it could muster 41 He 111s but fuel shortages reduced flying time and at Raffelding Austria, it was renamed I./KG(J) 27 and committed to Defence of the Reich duties from 23 November 1944.