An early proponent of mechanisation and armoured warfare, he was a Wehrmacht Heer army corps commander at the beginning of the war, leading his troops during the invasion of Poland and the Battle of France.

When the First World War began he was assigned to the Western Front, serving as a company commander and staff officer for several corps and armies.

[3] After the Blomberg–Fritsch affair in early 1938, the result of which was the subjugation of the Wehrmacht to dictator Adolf Hitler, and as the Sudetenland Crisis unfolded, Hoepner joined the Oster conspiracy.

Hoepner's role in the plan was to lead the 1st Light Division toward Berlin and seize key objectives against the SS forces in the city.

[2] On 30 March 1941, Hitler delivered a speech to about two hundred senior Wehrmacht officers where he laid out his plans for an ideological war of annihilation (Vernichtungskrieg [de]) against the Soviet Union.

[11] As a commander of the 4th Panzer Group, he issued a directive to his troops: The war against Russia is an important chapter in the struggle for existence of the German nation.

[18] Ultimately, the army group defeated the defending Soviet Northwestern Front, inflicting over 90,000 casualties and destroying more than 1,000 tanks and 1,000 aircraft, then advanced northeast of the Stalin line.

"[20] On 6 July 1941, Hoepner issued an order to his troops instructing them to treat the "loyal population" fairly, adding that "individual acts of sabotage should simply be charged to communists and Jews".

The staff and detachments 2 and 3 of Einsatzgruppe A, one of the mobile killing squads following the Wehrmacht into the occupied Soviet Union, were brought up to the Luga district with assistance from the army.

A compromise solution was worked out whereas the infantry would attack north from both sides of Lake Ilmen, while the Panzer Group would advance from its current position.

The 4th Panzer Group was to be the main attacking force, which reached south of the Neva River, where it was faced with strong Soviet counter-attacks.

Hoepner was confident that the clearing of the pocket and the advance on Moscow could be undertaken at the same time and viewed Kluge's actions as interference, leading to friction and "clashes" with his superior, as he wrote in a letter home on 6 October.

Heavy rains and onset of the rasputitsa (roadlessness) caused frequent damage to tracked vehicles and motor transport further hampering the advance.

In a letter home, Hoepner stated that just two weeks of the frozen ground would allow his troops to surround Moscow, not taking into account the stiffening Soviet resistance and the condition of his units.

[33] Lacking strength and mobility to conduct battles of encirclement, the Group undertook frontal assaults which proved increasingly costly.

[34] A lack of tanks, insufficient motor transport and a precarious supply situation, along with tenacious Red Army resistance and the air superiority achieved by Soviet fighters hampered the attack.

The attack by the 2nd Panzer Group on Tula and Kashira, 125 km (78 mi) south of Moscow, achieved only fleeting and precarious success, while Guderian vacillated between despair and optimism, depending on the situation at the front.

[36] Facing pressure from the German High Command, Kluge finally committed his weaker south flank to the attack on 1 December.

In the aftermath of the battle, Hoepner and Guderian blamed slow commitment of the south flank of the 4th Army to the attack for the German failure to reach Moscow.

With the outer defensive belt completed by 25 November, Moscow was a fortified position which the Wehrmacht lacked the strength to take in a frontal assault.

Such "blinkered thinking" on Hoepner's part was common among the German commanders in charge of the operation, which in Stahel's opinion "even before it began, made little practical sense".

Assuming that Hitler's permission was on the way and not wanting to risk the matter any longer, Hoepner ordered his troops to withdraw on 8 January 1942.

[42] Hitler directed that Hoepner be deprived of his pension and denied the right to wear his uniform and medals, contravening the law and Wehrmacht regulations.

Like other defendants, including Erwin von Witzleben, Hoepner was humiliated during the trial by being made to wear ill-fitting clothes, and not being allowed to have his false teeth.

Judge Roland Freisler berated Hoepner, but, in an extremely unusual move given his very aggressive personality, he objected to him being made to dress in such a way.

[46] Under the Nazi practice of Sippenhaft (collective punishment) Hoepner's wife, daughter, son (a major in the army), brother and sister were arrested.