During Operation Barbarossa, he commanded the 45th Infantry Division in one of the initial battles, the Defense of Brest Fortress. For his part in these battles, he would be awarded the Knight's Cross of the Iron Cross. In 1942, he was transferred to Slovakia to lead German military mission at Slovakian Ministry of Defence. He was released from his position after conflict with Minister General Ferdinand Čatloš. From 1944 until the war's end, he was the Chief of Special Staff II. After the war, Schlieper lived in Nürnberg until his death on 4 June 1977.