Short answers, pulled from the story.
Gerd von Rundstedt commanded Army Group South while Erich von Manstein served as his chief of staff. This force marched into Poland alongside Army Group North to seize territory for the Third Reich.
Hitler ordered forces to capture Soviet Ukraine and its capital city Kiev during Operation Barbarossa. The region held major centers of industry and mining that Germany needed along with good farmland required for living space plans.
In preparation for Case Blue the 1942 campaign split Army Group South into two separate formations. Army Group A received orders to move south toward the oil fields in the Caucasus while Adolf Hitler met with officers at the headquarters of Army Group South in Poltava on the 1st of June 1942.
On the 4th of April 1944 Army Group South changed its name to Army Group North Ukraine. This designation existed from the 4th of April until the 28th of September that same year before being renamed again in September 1944.
General Friessner led the main group while General Fretter-Pico commanded the Sixth Army in October 1944. Lieutenant General Kleeman directed IV Panzer Corps and Major General Vörös led VII Army Corps among other coordinated divisions including 24th Panzer Division and 76th Infantry Division.