On 10 August 1915, Greim transferred to the German Air Service (Fliegertruppe).
On 10 October 1915, while flying two-seaters in FFA 3b as an artillery spotting observer, Greim claimed his first aerial victory: a Farman.
He also served with FAA 204 over the Somme.
After undergoing pilot training, Greim joined FA 46b on 22 February 1917.
He transferred to Jagdstaffel 34 in April 1917.
He scored a kill on 25 May 1917, and on the same day he received the Iron Cross First Class.
On 19 June, he rose to command Jasta 34.
Greim became an ace on 16 August 1917, when he shot down a Sopwith 1 1/2 Strutter.
By 16 October, his victory tally totaled 7.
There was a lull in his successes until February 1918.
On the 11th, he had an unconfirmed victory and on the 18th he claimed aerial victory number 8.

On 21 March 1918, the day of his ninth credited victory, Greim became Commanding Officer of Jagdgruppe 10.
He flew with them until at least 18 June, when he notched up his 15th success.
On 27 June 1918, while Greim was engaging a Bristol Fighter, his aircraft lost its cowling.
The departing cowling damaged his top wing, along with the lower left interplane strut, but Greim managed to land the machine successfully.
By 7 August 1918 he was commanding Jagdgruppe 9, and scored his 16th victory.
On 23 August, he cooperated with Vizefeldwebel Johan Putz in what was arguably the first successful assault by aircraft on armored tanks.
On 27 September, he scored kill number 25 while flying with Jagdgruppe 9.

He returned to Jasta 34 in October 1918.
The Jasta had been re-equipped with 'cast-offs' from Richthofen's Flying Circus, Jagdgeschwader 1.
The new equipment was warmly welcomed as being superior to the older Albatros and Pfalz fighters that they had been previously equipped with.
Greim's final three victories came during this time, while he was flying Albatros D.Vs, Fokker Triplanes, and Fokker D.VIIs.
By the war's end he had scored 28 victories and had been awarded the Pour le Mérite on 8 October, as well as the Bavarian Military Order of Max Joseph (Militär-Max Joseph-Orden).
This latter award made him a Knight (Ritter), and allowed him to add both this honorific title and the style 'von' to his name.
Thus Robert Greim became Robert Ritter von Greim.
On 10 August 1915, Greim transferred to the German Air Service (Fliegertruppe). On 10 October 1915, while flying two-seaters in FFA 3b as an artillery spotting observer, Greim claimed his first aerial victory: a Farman. He also served with FAA 204 over the Somme. After undergoing pilot training, Greim joined FA 46b on 22 February 1917. He transferred to Jagdstaffel 34 in April 1917. He scored a kill on 25 May 1917, and on the same day he received the Iron Cross First Class. On 19 June, he rose to command Jasta 34. Greim became an ace on 16 August 1917, when he shot down a Sopwith 1 1/2 Strutter. By 16 October, his victory tally totaled 7. There was a lull in his successes until February 1918. On the 11th, he had an unconfirmed victory and on the 18th he claimed aerial victory number 8.
On 21 March 1918, the day of his ninth credited victory, Greim became Commanding Officer of Jagdgruppe 10. He flew with them until at least 18 June, when he notched up his 15th success. On 27 June 1918, while Greim was engaging a Bristol Fighter, his aircraft lost its cowling. The departing cowling damaged his top wing, along with the lower left interplane strut, but Greim managed to land the machine successfully. By 7 August 1918 he was commanding Jagdgruppe 9, and scored his 16th victory. On 23 August, he cooperated with Vizefeldwebel Johan Putz in what was arguably the first successful assault by aircraft on armored tanks. On 27 September, he scored kill number 25 while flying with Jagdgruppe 9.
He returned to Jasta 34 in October 1918. The Jasta had been re-equipped with 'cast-offs' from Richthofen's Flying Circus, Jagdgeschwader 1. The new equipment was warmly welcomed as being superior to the older Albatros and Pfalz fighters that they had been previously equipped with. Greim's final three victories came during this time, while he was flying Albatros D.Vs, Fokker Triplanes, and Fokker D.VIIs. By the war's end he had scored 28 victories and had been awarded the Pour le Mérite on 8 October, as well as the Bavarian Military Order of Max Joseph (Militär-Max Joseph-Orden). This latter award made him a Knight (Ritter), and allowed him to add both this honorific title and the style 'von' to his name. Thus Robert Greim became Robert Ritter von Greim.
Robert Ritter von Greim