The Ustaše were a Croat Fascist Party led by The Poglavnik, Dr. Ante Pavelić, which was in charge of the Independent Croat State during WW2.
One of the most brutal rulers among the Axis, Pavelić wanted a racially pure Croatia, which demanded the purging of Serbs, Romani People and Jews, but not Bosniaks, because the Poglavnik believed that they shared the same roots of the Croats, and Bosniak Islam, together with Croat Catholicism would preserve the Croat Identity.
The Serbs were hated by the Croats, because they thought the serbs were invading, non-pure people and their plans for the serbs were, as the Poglavnik himself said, to "Kill a third, expel a third, and convert a third."
Puška puca a top rice, grmi kao grom,
Sad Ustasa bojak bije za Hrvatski dom.
Puška puca krv se lije, dušman bježi klet,
A Ustaška hrabra vojska vrši zavjet svet.
Tjera smjelo dušmanina, s praga djedovskog,
I podiže staru slavu roda Hrvatskog.
Puška puca a top rice, barut miriše,
Mlad Ustaša na bojištu ranjen izdiše.
Ne plaši ga grom pušaka ni topova jek,
Dom, sloboda, vjerna ljubav, rani su mu lijek.
Oj Hrvatska zemljo mila, svice danak tvoj,
Sad Ustaska hrabra vojska za te bije boj.
PS.: Thanks to a friend of mine, Zdravko, for providing me the translation