Michał Zieliński’s song "Serce w plecaku" was written and arranged in 1933 because of a competition organised by a newspaper "Żołnierz Polski". Since then the song has accompanieda lot of soldiers on the battlefields but not only there.

The composition was the most popular during World War II when songs especially military patriotic and ones were sung very often all over Poland. The song encouraged people and gave hope.

"Serce w plecaku" was sung in concentration camps, by troops of partisans and in prisons. Several reprinting  of the song made at that time is a proof of its huge popularity. Because of the success, many people claimed to be the author of that very popular composition. As a result, Michał Zieliński was forced to save his rights in court and finally he was officially approved to be the author of the song.