The Triune Kingdom of Croatia, Slavonia and Dalmatia was part of Austria-Hungary during World War I. Its territory was administratively divided between the Austrian and Hungarian parts of the empire; Međimurje and Baranja were in the Hungarian part (Transleithania), the Kingdom of Croatia-Slavonia was a separate entity associated with the Hungarian Kingdom, Dalmatia and Istria were in the Austrian part (Cisleithania), while the town of Rijeka had semi-autonomous status.

The unification of Croat-inhabited territories was a fundamental problem that had not been resolved with the creation of Dual Monarchy in 1867. An excess of political problems within Austria-Hungary itself, exacerbated by the earlier Balkan Wars, led to a state of unrest, strikes, and series of assassinations within Croatia at the outbreak of World War I. Croatian policy amounted to either trying to find the best solution whilst staying within the empire (such as Trialism in Austria-Hungary or Austro-Slavism) or unifying with Serbia and Montenegro (namely Yugoslavism), thereby forming a South Slavic state. Political action was extremely limited during the war, with much of the power being held by the Austro-Hungarian military authorities. Calling for the creation of a unified South Slavic state was thus equated with treason.

During World War I, Croats fought mainly on the Serbian Front, the Eastern Front and the Italian Front, against Serbia, Russia, and Italy, respectively. A small number of Croats also fought on other fronts. Some Croats, mostly Croatian Americans or Austro-Hungarian Army deserters, fought on the side of the Allies. Croatia suffered great human and economic losses during the war, which were exacerbated by the outbreak of Spanish flu pandemic in 1918.Proleksis enciklopedija-Hrvati Although Croatians were less than 10% of the population, they accounted for approximately 13-14% of the Austro-Hungarian military conscripts. Croatian military losses likely amounted to 190,000 people, while some sources list about 137,000 military and 109,000 civilian casualties.https://hrcak.srce.hr/clanak/233073 Hrvatska u Prvom svjetskom ratu - Bojišta, stradanja, društvo (Croatia in WWI, Battlefields, losses, society); Filip Đukić, Marko Pavelić, Silvijo Šaur, in Croatian

With Austria-Hungary's imminent defeat and collapse, on 29 October 1918, the Croatian Parliament declared the country's secession from the empire. Croatia subsequently joined the short-lived State of Slovenes, Croats and Serbs, which was united with Serbia to form the Kingdom of Serbs, Croats and Slovenes on 1 December 1918.

Situation in Croatia

Outbreak of War

Inter-ethnic tensions in Croatia increased following the assassination of Archduke Franz Ferdinand in Sarajevo on 28 June.Agičić, p. 303 The evening of the assassination, Croats held commemorative parades and gatherings, carrying pictures of the fallen heir to the throne and Croatian flags adorned with black flora. The Frankists were angered by his assassination as Franz Ferdinand had expressed sympathy for Croatian interests in the past. The Economist-The Serbs and the Hapsburgs The Frankists used some minor provocation and the atmosphere of bitterness to orchestrate anti-Serb demonstrations. After someone threw stones at the commemorative parade, the party's followers demolished Serb-owned stores and cafés, as well as the gathering places of pro-Yugoslav politicians.Agičić, p. 305Karaula, p. 274 MPs of the ruling Croatian-Serbian Coalition were physically attacked.Karaula, p. 268.-269 Members of the Social Democratic Party of Croatia and Slavonia held a protest against this violence.Agičić, p. 306. At the same time with large anti-Serbian riots in Vienna, Budapest and serious incidents in Condominium of Bosnia and Herzegovina, where many were wounded and murdered, conflict with the Yugoslav oriented citizens and associations, and Serbian associations and clubs, occurred in many Croatia towns. It was then when the building of the Serbian society Dušan silni in Dubrovnik was destroyed. This was all triggered by the fact that Franz Ferdinand's assassin, Gavrilo Princip, was an ethnic Serb.Demonstracije u Dubrovniku, Ilustrovani list, no. 29, Year I, 18 July 1914, Zagreb Riots were reported in Zadar, Metković, Bjelovar, Virovitica and Konavle, where protesters burned the Serbian flag. In Đakovo and Slavonski Brod, riots become so violent that the Austro-Hungarian Army was asked to intervene.Agičić, p. 306. In addition, a curfew was imposed Petrinja. In Vukovar and Zemun, the police managed to prevent more clashes. Most Serbs in Croatia approved the assassination. Cases of provocation, such as showing images of King Peter, joy, insults and celebrations, were reported.Agičić, p. 309.-310Karaula, p. 276.-278 Fourteen Serbs were arrested in Zadar for celebrating the assassination.Karaula, p. 277. Some pro-Yugoslav politicians, such as Ante TrumbićKrležijana – Ante Trumbić and Frano Supilo,Hrvatska enciklopedija (LZMK) – Frane Supilo decided to emigrate to avoid prosecution. The Croatian sculptor and architect Ivan Meštrović emigrated to the United States.Krležijana – Ivan Meštrović

An extraordinary session of the Croatian Parliament, which was convened to read a loyalty oath to the Empire and statement of condolence, was interrupted with accusations and recriminations against ruling Croatian-Serbian coalition by the Frankists. The Frankist politician Ivo Frank allegedly threatened the Serb representatives with a revolver.Agičić, p. 313 On 26 July, partial mobilization was announced. The next day, ban Iván Skerlecz issued a special regulation abrogating Ivan Mažuranić's 1875 Law on Freedom of Assembly. After war was declared on 28 July, many pro-war demonstrations were held in Zagreb. The arrival of recruits to the Zagreb barracks was welcomed with approval. People shouted to the soldiers: "Long live the Croatian army", to which the soldiers would respond: "Long live Croatia, long live the Croatian King [Franz Joseph]".Agičić, p. 313

Since the last conflict that Croatia was involved had taken place in 1878, and since all of Europe had enjoyed almost 50 years of peace and relative stability, the war was met with shock and disbelief.Kovačić, Krešimir; 1963., Clochemerle u Zagrebu – humoristični zapisi iz vremena prošlih Food prices immediately increased so the government had to freeze them. All associations except firefighting and charity were closed. Shortly thereafter, many pro-Yugoslav politicians and activists, such as Ivo Vojnović, Ivo Andrić, Budislav Grga Angjelinović and Oscar Tartaglia, were arrested. Most were freed in 1915, and no later than 1917, following the Emperor Charles' adoption of a general amnesty for political prisoners. He amnestied all political prisoners after coming under strong pressure from the international public and in preparation for the conclusion of a separate peace treaty with the Triple Entente. Most of the territory of modern-day Croatia, with the exception of Eastern Syrmia and some Dalmatian islands (Palagruža, Lastovo), was not affected by military operations, but 60,000 residents of Pula and the surrounding area were forced to relocate in 1915 to refugee camps in Austria, Bohemia and Slovenia because of the proximity of war, and the security of the naval port. Many of them died in those camps of diseases and hunger. In Istria, martial law was imposed. The Kingdom of Croatia-Slavonia had a difficult political situation. During all four years of war, Kingdom was in danger of seduction of the dictatorship of Military Commissariat which was constantly trying to seize power from the Croatian Parliament. The Frankists supported dictatorship because they hoped that they would become its levers.Karaula, p. 270.

The economic situation was very bad since nearly a million people from Croatia and Bosnia and Herzegovina was mobilized.Matica hrvatska-Prvi svjetski rat kao terra incognita Nevertheless, the involvement of Russian and Serbian prisoners of war, who were ascribed to agricultural estates and factories for forced labor, was helping out. There was a shortage of products, as evident from the fact that the copper roofs of Dubrovnik Cathedral and Croatian State Archives palace were removed and warm in the ammunition. Same fate befell the Jaruga Hydroelectric Power Plant on the river Krka near Šibenik. The bread was received only with food stamps, while textile goods, even those of nettles, were completely gone. There was so little vegetable that there were no food stamps for it, so potatoes and beans were given away by the government from time to time. Women and children would wait all night at the Ban Jelačić and Preradović squares to get their  kg of potatoes or beans that were given per person.Kovačić, Krešimir; 1963., Clochemerle u Zagrebu – humoristični zapisi iz vremena prošlih

Charity campaigns

Charity campaign I Gave Gold for Iron () was conducted in 1914. People donated their gold jewelry in exchange for iron rings. Donated gold was used for supporting families of killed soldiers.Pedagoška djelatnost Hrvatskoga povijesnog muzeja-"Dadoh zlato za željezo"-Hrvatska u Prvom svjetskom ratu 1914.–1918.

In October 1914, the Government Commissioner for the Organization of Voluntary War Nurse Service, with the approval of High Royal Government, started campaign Gold for Iron () in order to gather "gifts in the form of golden items, cash money, and other valuables". After the assessment and with the written confirmation, donors were given an iron ring worth as the given donation, with the minimum value of the donation being 5 krones. The purpose of this charity campaign was to raise money for the families of killed soldiers "who are natives of these Kingdoms and who have left their families with lack of supplies." The center of the organization was in Zagreb, while collecting was carried out at the premises of the First Croatian Savings Bank on Ilica no. 5, in its subsidiaries or by an authorized person in the smaller towns. The rings had inscription "I Gave Gold for Iron 1914" and were protected by the stamp of the Commission for the Organization of Voluntary War Nurse Service. In addition, in the year 1915, citizens of Zagreb could hammer a nail into the trunk of the lime tree at Nikola Šubić Zrinski Square for a symbolic amount of money, thus collecting resources for families of soldiers who were killed in the war.THE ESTABLISHMENT OF MEDICAL SCHOOL IN ZAGREB IN WORLD WAR I This action took place in other Croatian cities as well; in Pula donors could hammer a nail into the lighthouse, and in Jastrebarsko in wooden hawk.

Avenue of chestnut trees in the main street of Sinj, which is used as the racecourse of Sinjska alka, was planted in memory of the recruited soldiers who went to war in 1917. The number of trees equals the number of men who were recruited.Turistička zajednica Splitsko-dalmatinske županije-Sinj, drvored kestena Many members of the nobility were engaged in humanitarian work as well. Marija Janković, married Adamović, a noblewoman from Janković family, donated a large portion of her wealth for buying medication for poor. She was also personally helping wounded soldiers in Drenovci.

Taking care of starving children