[4] During the Italian fascist administration of Istria, he was one of the few who promoted the rights of the non-Italian population, that is the Croatian and Slovene people, who were threatened with assimilation at best, and persecuted at worst.

At the end of the 1920s, he lived and worked in Trieste, where he published the last permitted literature and had a prominent role in the Istrian resistance in the joint Slovene-Croatian organization Edinost.

Božo Milanović, Zvonimir Brumnić and other Croatian priests were one of the main arguments why Istria belongs to Croatia, (at the time part of Yugoslavia).

The main document that was used in Paris by the priests was the Spomenica hrvatskog svećenstva u Istri Savezničkoj komisiji za razgraničenje Julijske krajine.

Pavla za Istru ("Choir of St. Paul's Priests for Istria") and was signed by President Tomo Banko, Secretary Miro Bulešić, councilors Božo Milanović, Leopold Jurca, Josip Pavlišić, Antun Cukarić and Srećko Štifanić, as well as 48 board members.

After Istria passed to Yugoslavia, many (Croatian) Istrian populists (such as Mate Peteh), as well as priests, were targeted by the Yugoslavian authorities.