Supilo played the main role in changing the public opinion, which expressed itself in several elections that brought down the Autonomous Party (pro-Italian) and Serbian coalition that had gained power in the municipality of Dubrovnik in the 1880s with the support of the Habsburg court, which followed the policy of divide et impera.

Supilo's Rijeka and Zagreb faze have historically and continuously been analysed only when the studies and books have started to appear after the fall of SFRJ - that is because up until then his work has been continuously treated as a visionary activity towards his hearts agenda, and even back then in Yugoslavia (communistic), a serious dream of the creation of a united country of Croats and Serbs.

A good example of the newest appreciation of Supilo's political activity is a series of shorter texts published in the magazine Kolo, 1998., nm.

That, from the modern perspective an unusual idea, was a sign of the late national crystallisation on the Croatian side (in less parts Serbs and Slovenes - not to mention Macedonians and Montenegrins), as well as the territorial bubbles of Serbs and Croats, together with the fact Croatian and Serbian traditional languages have been exquisitely similar and inter - understandable on the conversational level.

[7] A much stronger change has been signified by the negotiations between the coalition and Supilo with the Hungarian and Italian politicians from Austro-Hungarian monarchy.

[2] But he has made a mistake in his prognosis: the Italian and Hungarian imperialism was so strongly and deeply rooted into the consciences of the nationalistic elites of those people that only the world wars were able to crumble down those views.

His radical approach brings him into a conflict with the leadership of the Coalition, which wanted a more careful politic, in order not to lose the ability of returning to power.

[4] The coalition has since then, and up until the fall of monarchy and creation of the Kingdom of Serbs, Croatian and Slovenes (1918), fallen down on the weapon in hand of the most influential Croatian pragmatic politician of the time, proponent of unitarism Svetozar Pribićević, whose primary goal was the wait of the fall of the monarchy and definite union with the Serbs.

In practice, it meant the creation of a coalition that virtually recognized Serbs as a political entity in Croatia for the first time in its history.

[citation needed] An even stronger turn was the negotiation of the coalition and Supilo with the Hungarian and Italian politicians from Austria-Hungary.

[1][11] After the assassination of Archduke Franz Ferdinand of Austria, he fled to Florence, Italy where on 22 November 1914 he formed the Yugoslav Committee with Ante Trumbić and Ivan Meštrović to lobby for independence from Austria-Hungary.

[2][5] He resigned from the committee in June 1916, but endorsed the Declaration of Corfu that created the Kingdom of Serbs, Croats and Slovenes.

[5][16][17] His death was immortalized by Krleža in his Ballads of Petrica Kerempuh: "Sopilovog Frana,/kem serce pregrizla horvacka je rana" (Frano Supilo, whose heart was eaten by the Croatian wound).