By the end of the war he was serving of the staff of Rüdiger von der Goltz, commander of the Baltic Sea Division.

[2][Note 2] On the outbreak of World War II, Küchler's district headquarters was designated as the Wehrmacht's 3rd Army.

[2] During the invasion of Poland, some of Küchler's troops captured Danzig while the bulk of his forces advanced against the Polish Modlin Army.

[5] Küchler refused to use his soldiers to persecute Jewish and Polish civilians, explaining to the Gauleiter of East Prussia Erich Koch that the "German army is not a supplier for a killer gang".

[5] On the morning of 10 May 1940, the German armed forces commenced the implementation of the Fall Gelb plan for the invasion of the Low Countries and France.

[9] The Luftwaffe started bombing Dutch airfields and other targets, with the city of Rotterdam in particular suffering a devastating assault that ended after four days with the old-city center entirely destroyed.

[10] Küchler, fighting under General Fedor von Bock and commanding the 18th Army, defeated the Dutch ground forces at Moerdijk, Rotterdam, and the Hague.

In 1940 he was supportive of Nazi racial policy and ordered on 22 February a halt to any criticism of "ethnic struggle being carried out in the General Government, for instance, that of the Polish minorities, of the Jews and those regarding Church matters".

After meeting Hitler in March 1941 to plan for Operation Barbarossa, Küchler told his divisional commanders on 25 April 1941:"We are separated from Russia, ideologically and racially, by a deep abyss.

During Operation Barbarossa, the 18th Army forced its way to Ostrov and Pskov after the Soviet troops of the Northwestern Front retreated towards Leningrad.

On 10 July 1941, both Ostrov and Pskov were captured and the 18th Army reached Narva and Kingisepp, from where advance toward Leningrad continued from the Luga River line.

This had the effect of creating siege positions from the Gulf of Finland to Lake Ladoga, with the eventual aim of isolating Leningrad from all directions.

In December 1941, with his express consent, units of the SD shot 240 mental patients in the Russian town of Makaryevo.

Brought back to Hitler's headquarters on 31 January 1944, Küchler was relieved of his command and replaced by Generaloberst Model.

In his testimony regarding the crimes against the Soviet prisoners of war, Küchler admitted that the conditions in the POW camps were harsh, but insisted that the main cause of that was the winter conditions of 1941–42, which he called an "act of God" and insisted that the army exaggerated POW mortality in their reports in an effort to receive more supplies for the prisoners.