Children of war, they broke off the nipples on the wheels of German cars, disabling batteries, counted enemy echelons, scouted - where, how many Germans, what firing points, what weapons. The information was passed on to the partisans. Trains were derailed, commandant's offices were burning, blown up cars were flying into ditches. Young underground fighters fought the enemy as best they could.

They organised their raids so that suspicion would not fall on their native village, so they went far into the forests.
Their «mischief» did not go unnoticed not only by the fascists, but also by their own people. The boys began to receive specific tasks and acted in an organised, more active and effective manner. They distributed leaflets, collected weapons and ammunition, smuggled explosives, called the right people, collected information about the number of German troops in the area, information about passing railway trains and passed it all to the partisans.

But there was a traitor in the village. It was impossible to stay in the native village. They had to leave. So Alexei Sergeyev became a fighter of the 4th partisan regiment, the 5th Leningrad partisan brigade.

During the partisan struggle against Nazi invaders, Alexei participated in the defeat of Nazi garrisons at the railway stations Lemenka, Morino (1943). In 1943 he received his first and the most dear to him award - the medal «For combat merits». In January 1944 he carried out the task of the partisan command to undermine the railway bed («Rail War»), which prevented the delivery of troops and equipment to Leningrad. Together with his comrades and troops of the Volkhov Front he participated in breaking the blockade of Leningrad.

Three years later Alexei Nikolayevich was unanimously elected secretary of the regiment's Komsomol bureau. Then he became deputy company commander, and in 1956, with the rank of captain, he headed the radio-technical company in Nefedovo, Vologda District.