A former reserve NCO of the Austro-Hungarian Army, he volunteered for the Polish forces fighting in the battle of Lwów in the Polish-Ukrainian War.
After the successful defense of the city, he returned to civilian life only to rejoin the Polish Army after the outbreak of the Polish-Soviet War.
During the Budennyi's assault on Lwów he was chosen as a commander of a volunteer battalion of roughly 330 men, mostly pupils and students.
[1] After their defenses were finally broken, Zajączkowski committed suicide not to fall into enemy hands.
For his merits he was posthumously promoted to the rank of Major and awarded with Virtuti Militari medal.