At the start of World War II, Kühn was in command of a tank training school near Berlin, where he had been since 10 November 1938.

[1] Kühn and his soldiers played a key part in the second stage of the battle, Fall Rot, when they were able to penetrate the French Weygand line on the first day of the offensive to a depth of 15 kilometers, destroyed fourteen artillery batteries and paved the way for the exploitation of the breakthrough.

The division then turned south and participated in the Battle of Rostov (1941), before spending the winter in defensive positions around the Mius River.

[1] In 1942, Kühn and his division were ordered to assist in the counter-offensive against Russian forces in the Izyum area, which is where the unit was located until the middle of May 1942.

On 23 February 1943, per orders directly from Adolf Hitler, he was put in charge of all motorization matters for all of the Wehrmacht.