 Complete the following phrase:  At the beginning of World War II in September 1939, Mackensen served as the chief of staff of the German 14th Army in the invasion of Poland. Later, he was made chief of staff of the 12th Army that fought in the Battle of France. On 1 January 1940, he was promoted to lieutet general and eight months later to General der Kavallerie. On 15 January 1941, Mackensen was made commanding general of III Army Corps under the 1st Panzer Army in Army Group South, and on 27 July 1941, received the Knights Cross of the Iron Cross. Mackensens forces were the first to reach Kiev at the 