When the attack on Poland was launched, a part of the 3rd Army moved toward the Polish Corridor and met with Kluge's 4th Army. Both the 3rd and 4th Armies implemented their plans well, and the Polish Campaign ended in victorious triumph for the German Army. The Red Army had attacked Poland from the east with a million men and advanced westwards to meet with the German troops, despite casualties that numbered more than expected. In Brest-Litovsk, a joint German and Soviet victory parade was held.
On November 5, 1939, only about five weeks following the end of the Polish Campaign, the Third Army was disbanded. The Third Army became one of the first German armies of World War II to be disbanded. Immediately after the Third Army was disbanded Küchler became commander of the newly formed 18th Army and led it during the Western Campaign of 1940. He stayed in that position until 1942, when he became commander of Army Group North to lead the Siege on Leningrad. That year he became generalfeldmarschall but was replaced in 1944 following the Soviet breakthrough against Army Group North.