When World War I started, Immelmann was called to active service, transferred to Die Fliegertruppe (later known as the Luftstreitkräfte) and was sent for pilot training at Johannisthal Air Field in November 1914.
 He was initially stationed in northern France.

Immelmann served as a pilot with Feldflieger Abteilung (Field Flier Detachment) 10 from February to April 1915, and then in FFA 62 by early May 1915.
On several occasions he engaged in combat while flying the L.V.G. two-seaters with which his units were equipped, but never with any success.
On 3 June 1915, he was shot down by a French pilot, but managed to land safely behind German lines.
Immelmann was decorated with the Iron Cross, Second Class, for preserving his aircraft.

Two very early examples of the Fokker Eindecker fighters were delivered to the unit, one Fokker M.5K/MG production prototype numbered E.3/15 for Oswald Boelcke's use, with Immelmann later in July receiving E.13/15 as a production Fokker E.I for his own use before the end of July.
It was with the E.13/15 aircraft, armed with the synchronized MG 08 machine gun, that he gained his first confirmed air victory of the war on 1 August 1915, a fortnight after Leutnant Kurt Wintgens obtained the very first confirmed German aerial victory on 15 July 1915 with his own Fokker M.5K/MG production prototype E.5/15 Eindecker, one of five built, following two unconfirmed ones on 1 and 4 July, all before Immelmann:
Like a hawk, I dived ... and fired my machine gun.
For a moment, I believed I would fly right into him.
I had fired about 60 shots when my gun jammed.
That was awkward, for to clear the jam I needed both hands — I had to fly completely without hands ...
Lieutenant William Reid fought back valiantly, flying with his left hand and firing a pistol with his right.
Nonetheless, the 450 bullets fired at him took their effect; Reid suffered four wounds in his left arm, and his airplane's engine quit, causing a crash landing.
The unarmed Immelmann landed nearby, and approached Reid; they shook hands and Immelmann said to the British pilot "You are my prisoner" and pulled Reid out of the wreckage and rendered first aid.

Immelmann became one of the first German fighter pilots, quickly building an impressive score of air victories.
During September, three more victories followed, and then in October he became solely responsible for the air defense of the city of Lille.
Immelmann became known as The Eagle of Lille (Der Adler von Lille).

Immelmann flirted with the position of Germany's leading ace, trading that spot off with Oswald Boelcke, another pioneer ace.
Having come second to Boelcke for his sixth victory, he was second to be awarded the Royal House Order of Hohenzollern for this feat.
On 15 December, Immelmann shot down his seventh British plane and moved into an unchallenged lead in the competition to be Germany's leading ace.

Immelmann was the first pilot to be awarded the Pour le Mérite, Germany's highest military honour, receiving it on the day of his eighth win, 12 January 1916.
The medal became unofficially known as the "Blue Max" in the German Air Service in honor of Immelmann.
His medal was presented by Kaiser Wilhelm II on 12 January 1916.
Oswald Boelcke received his medal at the same time.

Boelcke scored again two days later.
Immelmann would chase him in the ace race for the next four months, drawing even on 13 March at 11 each, losing the lead on the 19th, regaining it on Easter Sunday (23 April) 14 to 13, losing it again forever on 1 May. It was about this time, on 25 April, that Immelmann received a salutary lesson in the improvement of British aircraft.
As the German ace described his attack on two Airco DH.2s, "The two worked splendidly together ... and put 11 shots into my machine.
The petrol tank, the struts on the fuselage, the undercarriage and the propeller were hit ... It was not a nice business."

On 31 May, Immelmann, Max von Mulzer, and another German pilot attacked a formation of seven British aircraft.
Immelmann was flying a two-gun Fokker E.IV, and when he opened fire, the synchro
When World War I started, Immelmann was called to active service, transferred to Die Fliegertruppe (later known as the Luftstreitkräfte) and was sent for pilot training at Johannisthal Air Field in November 1914.  He was initially stationed in northern France.
Immelmann served as a pilot with Feldflieger Abteilung (Field Flier Detachment) 10 from February to April 1915, and then in FFA 62 by early May 1915. On several occasions he engaged in combat while flying the L.V.G. two-seaters with which his units were equipped, but never with any success. On 3 June 1915, he was shot down by a French pilot, but managed to land safely behind German lines. Immelmann was decorated with the Iron Cross, Second Class, for preserving his aircraft.
Two very early examples of the Fokker Eindecker fighters were delivered to the unit, one Fokker M.5K/MG production prototype numbered E.3/15 for Oswald Boelcke's use, with Immelmann later in July receiving E.13/15 as a production Fokker E.I for his own use before the end of July. It was with the E.13/15 aircraft, armed with the synchronized MG 08 machine gun, that he gained his first confirmed air victory of the war on 1 August 1915, a fortnight after Leutnant Kurt Wintgens obtained the very first confirmed German aerial victory on 15 July 1915 with his own Fokker M.5K/MG production prototype E.5/15 Eindecker, one of five built, following two unconfirmed ones on 1 and 4 July, all before Immelmann:
Like a hawk, I dived ... and fired my machine gun. For a moment, I believed I would fly right into him. I had fired about 60 shots when my gun jammed. That was awkward, for to clear the jam I needed both hands — I had to fly completely without hands ...
Lieutenant William Reid fought back valiantly, flying with his left hand and firing a pistol with his right. Nonetheless, the 450 bullets fired at him took their effect; Reid suffered four wounds in his left arm, and his airplane's engine quit, causing a crash landing. The unarmed Immelmann landed nearby, and approached Reid; they shook hands and Immelmann said to the British pilot "You are my prisoner" and pulled Reid out of the wreckage and rendered first aid.
Immelmann became one of the first German fighter pilots, quickly building an impressive score of air victories. During September, three more victories followed, and then in October he became solely responsible for the air defense of the city of Lille. Immelmann became known as The Eagle of Lille (Der Adler von Lille).
Immelmann flirted with the position of Germany's leading ace, trading that spot off with Oswald Boelcke, another pioneer ace. Having come second to Boelcke for his sixth victory, he was second to be awarded the Royal House Order of Hohenzollern for this feat. On 15 December, Immelmann shot down his seventh British plane and moved into an unchallenged lead in the competition to be Germany's leading ace.
Immelmann was the first pilot to be awarded the Pour le Mérite, Germany's highest military honour, receiving it on the day of his eighth win, 12 January 1916. The medal became unofficially known as the "Blue Max" in the German Air Service in honor of Immelmann. His medal was presented by Kaiser Wilhelm II on 12 January 1916. Oswald Boelcke received his medal at the same time.
Boelcke scored again two days later. Immelmann would chase him in the ace race for the next four months, drawing even on 13 March at 11 each, losing the lead on the 19th, regaining it on Easter Sunday (23 April) 14 to 13, losing it again forever on 1 May. It was about this time, on 25 April, that Immelmann received a salutary lesson in the improvement of British aircraft. As the German ace described his attack on two Airco DH.2s, "The two worked splendidly together ... and put 11 shots into my machine. The petrol tank, the struts on the fuselage, the undercarriage and the propeller were hit ... It was not a nice business."
On 31 May, Immelmann, Max von Mulzer, and another German pilot attacked a formation of seven British aircraft. Immelmann was flying a two-gun Fokker E.IV, and when he opened fire, the synchro
Max Immelmann