Immediately following the German invasion of Poland, Greiser was transferred from Danzig and appointed Chef der Zivilverwaltung im Militärbezirk Posen or Chief of Civil Administration in the military district of Posen, which was annexed to the German Reich on 8 October 1939.
The military administration ended and he was then appointed Gauleiter (21 October) and  Reichsstatthalter für den Reichsgau Posen (2 November).
On 29 January 1940, the region was renamed Reichsgau Wartheland.
On 30 January 1942, Greiser was promoted to SS-Obergruppenführer and on 16 November he was named Reich Defense Commissioner for his Gau.

The territory was potentially very rich – the Prussian Imperial province of Posen had been the breadbasket of Wilhelmine Germany before 1914, possessed an excellent rail and road network, and a comparatively healthy and well educated workforce; Litzmanstadt (Łódź) had developed a fairly sophisticated industrial base during the 19th century.
Although every Gauleiter was expected to fully Germanize his assigned area by any means, Greiser emphasized brutality to achieve this goal.
He was an ardent racist who enthusiastically pursued an 'ethnic cleansing' program to rid the Warthegau of Poles and to resettle the 'cleansed' areas with ethnic Germans.
This was along the lines of the racial theories espoused by Reichsführer-SS Heinrich Himmler.
Mass expulsions of Poles from the Warthegau to the General Government and summary executions were the norm.
A Polish servant in Greiser's house described him as "a powerfully built figure.
He was a tall man, you could see his arrogance, his conceit.
He was so vain, so full of himself—as if there was nothing above him, a god, almost.
Everybody tried to get out of his way, people had to bow to him, salute him.
And the Poles, he treated them with great contempt.
For him the Poles were slaves, good for nothing but work".
Greiser himself stated his beliefs: "If, in past times, other peoples enjoyed their century-long history by living well, and doing so by getting foreign peoples to work for them without compensating them accordingly and without meting out justice to them, then we too, as Germans want to learn from this history.
No longer must we stand in the wings; on the contrary, we must altogether become a master race!".

In addition to mass deportation, Greiser's district was also at the forefront of "internal" racial cleansing according to Nazi ideals.
His subordinate Wilhelm Koppe provided the 'Special Detachment (Sonderkommando) Lange' to the nearby Gau of East Prussia during May and June 1940.
This SS squad gassed 1558 patients from mental asylums at the Soldau concentration camp and then returned to his region to continue this process.

Greiser was involved in the resettlement of German refugees from lands annexed to the Soviet Union in 1939 and 1940.
Between October and December 1939, nearly 60,000 Volksdeutsche (ethnic Germans) arrived in Germany from the Baltic states of Estonia and Latvia.
Evidently Wilfried Strik-Strikfeldt (later employed as translator for General Andrey Vlasov) was in this group, as he "resettled" in Posen.
Neighbouring Gauleiter and rival Albert Forster refused them entry, and they were largely settled in properties seized from Poles in Poznań and across the Wartheland.
However even Greiser was wary, noting that many were elderly and urbanized aristocrats with a strong class consciousness, not the virile peasant warrior types idolized by the SS.
Closer to his heart were the over 100,000 ethnic Germans who were evacuated from Volhynia and eastern Galicia.
These were mostly farmers and rural people, and, learning from the Baltic experience, Łódź in eastern Wartheland was designated the main Volksdeutsche Mittelstelle (VoMi) reception centre.
In May 1940 a further 30,000 Volksdeutsche were relocated from the Nazi General Government of Poland to Greiser's domain.
After 1941 a further 300,000 ethnic Germans were evacuated from Russia and Ukraine to Wartheland during the German invasion and occupation of the Soviet Union.
Greiser's Poznań was considered t
Immediately following the German invasion of Poland, Greiser was transferred from Danzig and appointed Chef der Zivilverwaltung im Militärbezirk Posen or Chief of Civil Administration in the military district of Posen, which was annexed to the German Reich on 8 October 1939. The military administration ended and he was then appointed Gauleiter (21 October) and  Reichsstatthalter für den Reichsgau Posen (2 November). On 29 January 1940, the region was renamed Reichsgau Wartheland. On 30 January 1942, Greiser was promoted to SS-Obergruppenführer and on 16 November he was named Reich Defense Commissioner for his Gau. 
The territory was potentially very rich – the Prussian Imperial province of Posen had been the breadbasket of Wilhelmine Germany before 1914, possessed an excellent rail and road network, and a comparatively healthy and well educated workforce; Litzmanstadt (Łódź) had developed a fairly sophisticated industrial base during the 19th century. Although every Gauleiter was expected to fully Germanize his assigned area by any means, Greiser emphasized brutality to achieve this goal. He was an ardent racist who enthusiastically pursued an 'ethnic cleansing' program to rid the Warthegau of Poles and to resettle the 'cleansed' areas with ethnic Germans. This was along the lines of the racial theories espoused by Reichsführer-SS Heinrich Himmler. Mass expulsions of Poles from the Warthegau to the General Government and summary executions were the norm. A Polish servant in Greiser's house described him as "a powerfully built figure. He was a tall man, you could see his arrogance, his conceit. He was so vain, so full of himself—as if there was nothing above him, a god, almost. Everybody tried to get out of his way, people had to bow to him, salute him. And the Poles, he treated them with great contempt. For him the Poles were slaves, good for nothing but work". Greiser himself stated his beliefs: "If, in past times, other peoples enjoyed their century-long history by living well, and doing so by getting foreign peoples to work for them without compensating them accordingly and without meting out justice to them, then we too, as Germans want to learn from this history. No longer must we stand in the wings; on the contrary, we must altogether become a master race!".
In addition to mass deportation, Greiser's district was also at the forefront of "internal" racial cleansing according to Nazi ideals. His subordinate Wilhelm Koppe provided the 'Special Detachment (Sonderkommando) Lange' to the nearby Gau of East Prussia during May and June 1940. This SS squad gassed 1558 patients from mental asylums at the Soldau concentration camp and then returned to his region to continue this process.
Greiser was involved in the resettlement of German refugees from lands annexed to the Soviet Union in 1939 and 1940. Between October and December 1939, nearly 60,000 Volksdeutsche (ethnic Germans) arrived in Germany from the Baltic states of Estonia and Latvia. Evidently Wilfried Strik-Strikfeldt (later employed as translator for General Andrey Vlasov) was in this group, as he "resettled" in Posen. Neighbouring Gauleiter and rival Albert Forster refused them entry, and they were largely settled in properties seized from Poles in Poznań and across the Wartheland. However even Greiser was wary, noting that many were elderly and urbanized aristocrats with a strong class consciousness, not the virile peasant warrior types idolized by the SS. Closer to his heart were the over 100,000 ethnic Germans who were evacuated from Volhynia and eastern Galicia. These were mostly farmers and rural people, and, learning from the Baltic experience, Łódź in eastern Wartheland was designated the main Volksdeutsche Mittelstelle (VoMi) reception centre. In May 1940 a further 30,000 Volksdeutsche were relocated from the Nazi General Government of Poland to Greiser's domain. After 1941 a further 300,000 ethnic Germans were evacuated from Russia and Ukraine to Wartheland during the German invasion and occupation of the Soviet Union. Greiser's Poznań was considered t
Arthur Greiser