Short answers, pulled from the story.
The German military renamed Army Group South to Army Group A on the 26th of October 1939. This new formation took its place along the France, Germany border and Belgium, Germany border within the Eifel and Hunsrück mountain ranges.
Gerd von Rundstedt served as commander while Erich von Manstein held the role of chief of staff for Army Group A. These two men had led the same unit during the Invasion of Poland in September and October 1939.
On the 7th of July 1942, Army Group South split into two new formations where Army Group A moved south toward the Caucasus. Wilhelm List commanded Army Group A initially before Adolf Hitler assumed direct command on the 9th of September 1942.
Some 340,000 British and Allied soldiers were successfully evacuated out of Dunkirk while leaving behind much heavy equipment including 475 tanks. The halting order was revoked on the 26th of May and Dunkirk captured on the 4th of June.
Hitler renamed three army groups ending the final deployment of Army Group A on the 25th of January 1945. It became the new Army Group Center while old Army Group Center became Army Group North and no subsequent Army Group A was formed after this redesignation.