In 1917, having graduated with distinction (Gold Medal), Stanisław Szostak was enrolled as an officer cadet (junker) in the Nikolayevskoye School of Engineers in Petrograd, Russia.

Following the unsuccessful counter-revolutionary Junker mutiny of 29 October 1917 he was imprisoned in the Fortress of Peter and Paul in Petrograd.

At the beginning of his tour of service, in January 1918 he took part in the capture of the Bobrujsk fortress occupied by the Bolshevik forces.

After completing the course he was assigned to serve with the reserve battalion of the 56 Wielkopolski Infantry Regiment as a platoon commander.

Then, in the years 1935 to 1938 he was the head of the Training Department at the headquarters of the Armoured Corps attached to the Ministry of Military Affairs in Warsaw.

[2] After the outbreak of World War II in Europe, Major Szostak and the 32nd Reconnaissance Armoured Unit under his command took part in the battles of Grajewo and Szczuczyn.

By 16 September he lost all his armour but fought the invading Soviets in Grodno and organised defence of the town of Giby.

On 25 August Major Szostak joined the so-called "Anders Army", a Polish corps that was being organised in the Soviet Union.

He was given the command of the Polish Armoured Forces in the Soviet Union on 20 September and remained in this post until the evacuation to Iran on 28 March 1942.

From 3 April to 14 May he was in charge of the Armoured Forces and Transport Service on the staff of gen. Boruta Spiechowicz (troops evacuated to Iran.)

Colonel Zebrowski on page 491 of his monograph writes: Major Szostak was responsible for the entire training of the armoured troops of the 2nd Polish Corps.