Concerning Franz Ferdinand and his trialist project, I will try to summarize his plans. One thing to note: Ferdinand's trialism project wasn't meant to stop Slav ambitions, no, trialism was meant to stop the ambitions of another influential people group: the Hungarians. Ferdinand considered the Compromise of 1867 a big mistake. In 1897, Hungary received some concessions regarding financial matters, but their main request was that the Hungarian language be given the status of an official language in the army in all military units stationed in Hungary. Certain Austrian officers serving in Hungary, including Franz Ferdinand himself, bitterly opposed this. In 1902 there was a proposal in the Hungarian Parliament. What was proposed? The formation of a separate Hungarian army and the introduction of the Hungarian language in the military. Franz Ferdinand advised Franz Joseph to use military force against the Parliament. The emperor accepted and on February 19 1906 a Honved battalion entered the Parliament and chased the delegates away. However, Franz Joseph wasn't willing to go any further, much to the disappointment of Franz Ferdinand. In 1907 relationship between Franz Joseph and Franz Ferdinand deteriorated. The emperor ordered Ferdinand to go to Budapest to celebrate the 40th anniversaty of the Compromise. The archduke protested. In a July 1907 letter to the emperor, Ferdinand claimed that Hungary was ruled by anti-Habsburgs. That same year, the archduke finally started preparing for his eventual ascent to the throne. Back then, the archduke and the Foreign Minister Aehrental were allies and their main concern was Hungary. However, the very next year, the pair drifted apart and Aehrental started supporting the policy of Franz Joseph i.e. giving concessions to the Hungarians. The Archduke wrote to the emperor on August 17 1909 saying that he cannot agree with Aehrental's plan to give Hungarians more power.
Ferdinand's support was in the military. One thing to note: Franz Ferdinand and Conrad von Hotzendorf WERE ALLIES, CLOSE ALLIES. Conrad only became the Chief of Staff in 1906 because the Archduke pushed for his appointment. This is how Alexander von Brosch-Aarenau described the relationship between Conrad and Ferdinand: „The Archduke found in Conrad a right man with whom he can cooperate in the matters regarding the creation of a single army. It was to the Archduke's credit that this able general was given a position that suits him.“ Conrad also sent a memorandum to the emperor in 1907 opposing any further concession to Hungary. In 1911, Conrad went too far and suggested a preventive war against Italy. The emperor dismissed him, but Franz Ferdinand bitterly opposed Conrad's dismissal and wrote a letter to the emperor begging the emperor not to dismiss Conrad. In this letter, the Archduke listed 4 main enemies of the Empire in his opinion: the Hungarians, Jews, Freemasons and socialists. Conrad was dismissed, but was reinstated the next year, once again because Franz Ferdinand intervened. Ferdinand's thoughts about Hungary are perhaps best shown in his letter to the emperor from December 18 1908: „Hungarian spirit, the entire Hungarian body, has always been the same as the latent opposition to the crown. Teacher „History“ as well as military history best proves that the Hungarian nation, whether under the Arpads, the Angevins or the Habsburgs, has always fought against the rights of the Crown and even struck deals with directly with the Crown's enemies.“ In August 1909 Ferdinand went further and said that all anti-Habsburg movements, in Bohemia, South Slavic parts of the Monarchy, Trieste etc. can be traced back to Hungary. Ferdinand's main supporter, apart from the military circles, was Ottokar Czernin. He was constantly advising the Archduke, telling him that if the Monarchy wanted to survive, a single, unified army must be created. Czernin believed that Hungary must lose its 1867 privileges and be reduced to the status of an ordinary province.
The Archduke also had supporters from other people groups of the Empire. There were Romanians and even Slovaks. Milan Hodža was in contact with the Archduke's Belvedere Circle (so called because the Archduke's main residence was Belvedere). Ferdinand was also a staunch absolutionist and always preferred to solve disputed by force. When in 1906 Franz Joseph sent the Hungarian Minister of Interior Gyula Andrassi to meet Ferdinand, Ferdinand told Andrassy that if his reform plans were not accepted, he would march into Hungary with the military. Ferdinand was also a huge anti-Semite. He despised Jews and this is what he had to say about the Jews of Hungary: „A Jew is always on the side of the strong: if it can bring him profit, he is even an anti-Semite.“ Ferdinand wanted to limit the freedom of the press in order to suppress liberal and socialist ideas. In 1913, the Archduke prepared for conflict with Hungary. He started withdrawing units made of Hungarians from Hungary and put units made up of Croatians in their place. That same year, Karl Tersztyánszky von Nádas became the head of the Budapest Corps. He was the Archduke's man.
So, what was the purpose of trialism? Weakening Hungary. Ferdinand knew that in 1848/49 the court in Vienna used non-Hungarians to fight the Hungarians and he wanted to repeat that. Ferdinand relied on the people groups: Romanians, Slovaks and Croats. Ferdinand started supporting trialism in 1903 and that same year he said as much to Nicholas II of Russia. Why Russia? Well, who was Austria's main ally in the fight against the Hungarians in 1848/49? Oh right, Russia. Ferdinand knew that there were many supporters of trialism, especially in Croatia. In Croatia the Party of Rights supported the idea, especially the supporters of Josip Frank. The Frankists were extremely anti-Serb so Ferdinand decided to use them in order to divide Serbs and Croats, while at the same time weakening Hungary. However, his plans for Croatia suffered a serious setback in 1905/06 after the publishing of the Resolution of Rijeka and the creation of Croat-Serb Coallition. The Resolution put forward a plan of Slavic-Hungarian cooperation against the hegemony of Vienna. This went against all of Ferdinand's plans. Soon after, in Dubrovnik and Herzegovina, there were protests against the Archduke. Frano Supilo was suspected of organizing the protests, but he claimed that the reactions of the people were spontaneous. After this, FERDINAND SECRETLY ABANDONED TRIALISM. But, he never denounced it in public. This is an excerpt from a letter sent by von Brosch-Aarenau to Ferdinand in 1910: „Who could guarantee that this new unit, consisting of Croatia, Bosnia, Dalmatia and Styria, which would cut Austria off from the sea, would really be loyal to the Emperor and support Austria and the interests of the dynasty? A new Rijeka Resolution could come overnight and the Croats are still in the hands of the Hungarian government.“ Theodor von Sosnosky, who was politically close to Ferdinand, said that Ferdinand only flirted with trialism and that his true dream was Greater Austria. Sosnosky said this: „The Archduke abandoned the idea of trialism because South Slavic resistance increased. He lost faith in Croatia.“