 Displeased with reforms in Poland that were threatening its influence there, Russia invaded Poland in May 1792, starting the Polish – Russian War of 1792. Two weeks before the war started, on May 4 Zajączek became the commander of the 3rd Front Guard Regiment (3 regiment straży przedniej). On May 26 he was given command over a reserve corps, and on 29 May he was promoted to the rank of major general. After gathering some troops near Lublin, he departed towards Dubno, which he reached on 7 June. He was one of the Polish commanders at the victorius battle of Zieleńce, for which he received the highest decoration of the Polish military, Virtuti Militari, becoming the fifth person to receive this award, instituted just that year. He took part in some minor skirmishes, but did not participate in any significant battle before Stanisław August Poniatowski surrendered to the enemy, joining the Targowica Confederation. The war ended without any decisive battles, with the Polish army still in the fighting condition, not suffering from any major defeat nor from lack of supplies. Angered at the king's betrayal, Zajączek was one of the main proponents of the plan to kidnap the king; the plan, however, never came to fruition. Like many other dissatisfied officers, including Prince Poniatowski and Tadeusz Kościuszko, Zajączek requested a discharge from the army; his resignation was accepted on 18 August.
