After graduating from the Eberhard-Ludwigs-Gymnasium in Stuttgart, Heim entered the Württemberg Army on 24 June 1914 as an officer candidate (Fahnenjunker) in Field Artillery Regiment No.

When World War I began shortly thereafter, Heim went into the field with his regiment and was commissioned a lieutenant (Leutnant) on 25 February 1915 (with a Patent of 23 June 1913).

Heim's XXXXVIII Panzer Corps was placed behind the Romanian 3rd Army at the beginning of the Soviet Operation Uranus "to check the enemy attack" along Paulus's left flank.

"Hitler made him a scapegoat and relieved him of his position...despite the obvious lack of fighting experience, equipment and strength in Heim's Rumanian and German divisions.

"[5][6] After this, in January 1943, Heim was, at Hitler's order, dismissed from the Army, arrested and placed in solitary confinement at Moabit, finally being released in April 1943, when he was transferred to a military hospital at Ulm.

He learned, unofficially, that Hitler had been unwilling to cast blame on the Romanians for the poor quality of their troops so a German scapegoat was needed.

The ill-prepared and ill-suited garrison endured heavy bombardment and full-scale assaults when the 3rd Canadian Infantry Division launched Operation Wellhit.