Heeresgruppe Süd was formed from Arbeitstab von Rundstedt. Arbeitstab von Rundstedt was a covername used to hide the formation of a new Heeresgruppe beyond the already existing Heeresgruppen of the Werhmacht. Knowledge of the formation of a new Heeresgruppe would likely clue the outside world into the fact that Germany was making plans for offensive operations against Poland. Initally Arbeitstab von Rundstedt consisted of only 3 men, commanding officer Gerd von Rundstedt, his chief of staff Erich von Manstein and chief of operations Günther Blumentritt. To further expand
the staff of the Heeresgruppe (still in disguise), 12.Armee (itself also just forming) was absorbed into Arbeitstab von Rundstedt. On August 24th, 1939, orders were recieved formally creating Heeresgruppe
Süd. Interestingly, this Heeresgruppe would be the first of four Heeresgruppen which would eventually be known by the name of Süd. Some of the changes in Heeresgruppen designations and formations will be charted via the evolution of this Heeresgruppe in particular via the links below.
On September 1st, 1939 Heeresgruppe Süd took part in the the short lived but hard fought Polish Campaign as the southern counterpart to Heeresgruppe Nord. These two Heeresgruppen controlled all of the units that fought in Poland. Heeresgruppe Süd, operating out of the region of Silesia, controlled those units responsible for the southern drive on Warsaw and for flank protection against possible Polish counterattacks from Galacia.
After the Campaign in Poland, on October 8th, 1939, Heeresgruppe Süd was named Oberbefehlshaber Ost. The commander of the Heeresgruppe (still Rundstedt) became the supreme military commander in the regions Heeresgruppe Süd occupied. On October 26th, 1939, Heeresgruppe Süd was moved to the west to the regions of Eifel/Hunsrück where it shortly after became Heeresgruppe A.
1939 Sep 8.Armee, 10.Armee, 14.Armee
1939 Oct Frontier Abschnitt Nord (3.Armee), Frontier Abschnitt Mitte (8.Armee, then 5.Armee), Frontier Abschnitt Süd (??)
Gen Gerd von Rundstedt