At the Versailles conference after World War I, Trumbić had to represent Yugoslav concerns in the face of Italian territorial ambitions in Dalmatia (temporarily settled in 1920, but raised again with Benito Mussolini).

In 1929, claiming to bring an end to the ongoing bickering between the Serbian and the Croatian representatives within the kingdom, King Alexander of Yugoslavia staged a coup d'état and banned all political parties, and removed the individual nationalities Serb and Croat from the bigger picture.

In a September 1932 interview with The Manchester Guardian Trumbić wondered whether Croatia should separate from the Kingdom of Yugoslavia and pursue a union with Austria.

[3] In November 1932 Trumbić edited the Zagreb Points, a series of demands put forth by the Peasant-Democratic coalition to counter Serbian hegemony.

According to Henri Pozzi, Trumbić later regretted the end of Austria-Hungary,[6] as the South Slav state he had helped to create proved incapable of his intended reforms.