As a Croatian nationalist, he was persecuted in the Kingdom of Yugoslavia, and his murder subsequently caused an internationally publicized affair.

[2] The family coat of arms was included in Der Adel von Kroatien und Slavonien (1899) as "Sufflay de Otrussevcz".

Tadija Smičiklas considered Šufflay his most gifted student and took him as his assistant when editing Codex Diplomaticus of the Yugoslavian Academy of Sciences and Arts.

[citation needed] This conviction clashed with the prevailing opinion of Croatian historians that the Croats were representatives of the West, as opposed to the Balkans.

The reaction to the sentence was stronger abroad than in Croatia, as scientific colleagues from numerous countries tried to obtain his release but without success.

In 1928, when Stjepan Radić was assassinated in the Yugoslav parliament, a year before king Alexander I would establish his dictatorship, Šufflay wrote Hrvatska u svijetlu svjetske historije i politike (Croatia in the Light of World History and Politics).

It is my thesis that the Croatian nation, as a citizen of the great empire of the western civilization, has the right to raise its voice against any oppression.

In Croatia, the Yugoslav idea is a shallow wreckage under which the Croatian national volcano boils; only a subtle push is necessary to make it erupt.

To me personally, as a philosopher and an open-minded Croat, it is the same whether I sit shackled at the court or a penitentiary, or whether I get out into the false freedom hiding the larger dungeon in which – thank God, only temporarily!

[10] On the request of the Albanian government and the Academy of Sciences in Vienna, he continued the work of Jireček and Thalloczy, editing the 3rd book of Codex albanicus, an archival collection.

The appeal was addressed to the Paris-based Ligue des droits de l'homme[11] (Human Rights League) and made the front page of the New York Times on 6 May 1931.