Shortly before the start of the war, Senior Lieutenant A.E. Potapov was assigned to the commander of the 333rd Infantry Regiment of the 6th Infantry Division and in late May 1941 was appointed senior adjutant (chief of staff) of one of the battalions of this regiment. Since October 1939, after the end of the campaign of Soviet troops in Poland, units of the 6th Infantry Division were stationed in the area of the city of Brest-Litovsk and adjacent areas north of the Mukhavets River, taking over the garrison service in Brest and guarding the state border along the Western Bug River in area of Brest. The barracks of the 333rd Infantry Regiment were located directly in the citadel of the Brest Fortress.
On the night of June 22, 1941, Potapov was on duty at the 333rd Infantry Regiment. With the first explosions of enemy shells and bombs, he raised the regiment on alert. The commander of the regimental school platoon, Lieutenant Naganov, was ordered to take up defensive positions in the area of the Terespol Gate. Realizing that the commander and commissar of the regiment would not be able to break through to the besieged fortress, Potapov led the defense in the area. Acting boldly and decisively, Senior Lieutenant A.E. Potapov and Assistant Chief of Staff of the 333rd Infantry Regiment, Lieutenant A.S. Sanin organized the fighters, thanks to which the enemy, who broke into the citadel, was met with organized fire and suffered heavy losses. Nearby there was a building that housed the 9th border outpost. Here fighters fought under the command of the chief of the outpost, Lieutenant A. M. Kizhevatov. On June 23, when only ruins remained of their building, Kizhevatov and his fighters moved to the basements of the barracks of the 333rd regiment and continued to lead the defense together with Potapov.
Around June 25, a group of fighters from the 132nd battalion of the NKVD convoy troops, who had previously defended in the location of their unit, also ran into the barracks of the 333rd regiment. The soldiers of this group carried with them the battle banner of their battalion, which later, during an attempt to break through, they hid in an air tube on the second floor, opposite the headquarters of the 132nd battalion.
Among the fighters included in the Potapov group, the young pupil of the musician platoon of the regiment Petya Klypa stood out for his courage and courage.
During the week, under the command of Potapov, Sanin and Kizhevatov, the defenders of the barracks of the 333rd Infantry Regiment fought off numerous enemy attacks. Women and children, who were hiding in the basements of the barracks with the soldiers, had to be taken prisoner on the 3rd or 4th day of the war. In the last days of the defense, Lieutenant Sanin fell seriously ill and lay half-delirious, not getting up (he was captured, according to his own recollections, on June 27, and according to the German prisoner of war card - on June 24).
On June 29, when the ammunition nearly ran out, it was decided to make a last desperate attempt to break through. It was supposed to break through not to the north, where the enemy was expecting attacks and kept large forces at the ready, but to the south, towards the Western Island, in order to then turn to the east, cross the Bug arm and pass the hospital on the Southern Island to get into the vicinity of Brest. This breakthrough ended in failure - most of its participants died or were captured. AE Potapov, who was the leader of the breakthrough group, died in this battle. Lieutenant A. M. Kizhevatov, covering the breakthrough, remained in the Citadel and also died in battle.
In his book "Brest Fortress" the writer S.S.Smirnov, referring to the memoirs of a number of participants in the defense of the Brest Fortress, including Peter Klypa, pointed out that A.E. Potapov, perhaps, did not die during the breakthrough, but was taken prisoner and spent some time in a prisoner of war camp in Biala Podlaska. It was even indicated that Potapov was one of the organizers of the mass escape of prisoners of war from this camp in September 1941. However, this information remains fundamentally inaccurate and unconfirmed.