The Croatian military was in a much worse state than that of the Serbs. In the early stages of the war, lack of military units meant that the Croatian Police force would take the brunt of the fighting. The Croatian National Guard, the new Croatian military, was formed on 11 April 1991, and gradually developed into the Croatian Army by 1993.
On 19 May 1991, the Croatian authorities held a referendum on independence with the option of remaining in Yugoslavia as a looser union. Serb local authorities issued calls for a boycott, which were largely followed by Croatian Serbs. The referendum passed with 94% in favor.
On 25 June 1991, both Slovenia and Croatia declared independence, which led to a short armed conflict in Slovenia called the Ten-Day War, and an all-out war in Croatia in the Croatian War of Independence in areas with a substantial ethnic Serb population.
On the morning of 26 June, units of the Yugoslav People's Army's 13th Corps left their barracks in Rijeka, Croatia, to move towards Slovenia's borders with Italy.
In August 1991, the Battle of Vukovar began. The Battle of Vukovar was an 87-day siege of Vukovar in eastern Croatia by the Yugoslav People's Army (JNA), supported by various paramilitary forces from Serbia, between August and November 1991. Before the Croatian War of Independence the Baroque town was a prosperous, mixed community of Croats, Serbs and other ethnic groups. As Yugoslavia began to break up, Serbia's President Slobodan Milošević and Croatia's President Franjo Tuđman began pursuing nationalist politics. In 1990, an armed insurrection was started by Croatian Serb militias, supported by the Serbian government and paramilitary groups, who seized control of Serb-populated areas of Croatia. The JNA began to intervene in favour of the rebellion, and conflict broke out in the eastern Croatian region of Slavonia in May 1991. In August, the JNA launched a full-scale attack against Croatian-held territory in eastern Slavonia, including Vukovar.
In September 1991, Croatian National Guard (ZNG) organised armed incursions across the Croatian border into Bosnia. ZNG opened mortar fire on Bosanska Dubica on 13 September 1991, and raided Bosanski Brod on 15 September 1991.
On 20 September 1991, the JNA transferred troops to the front at Vukovar via the Višegrad region of northeastern Bosnia. In response, local Croats and Bosniaks set up barricades and machine-gun posts. They halted a column of 60 JNA tanks but were dispersed by force the following day. More than 1,000 people had to flee the area. This action, nearly seven months before the start of the Bosnian War, caused the first casualties of the Yugoslav Wars in Bosnia.
On October 5, President Tuđman made a speech in which he called upon the whole population to mobilize and defend against "Greater Serbian imperialism" pursued by the Serb-led JNA, Serbian paramilitary formations, and rebel Serb forces.
The next day, as a previously agreed three-month moratorium on implementation of the declaration of independence expired, the Croatian Parliament severed all remaining ties with Yugoslavia. 8 October is now celebrated as Independence Day in Croatia.
Croatian forces made further advances in the second half of December, including Operation Orkan 91 which was a military offensive undertaken by the Croatian Army (Hrvatska vojska – HV) against the Yugoslav People's Army (Jugoslovenska Narodna Armija – JNA) and SAO Western Slavonia Territorial Defense Forces in the Sava River valley, in the region of Western Slavonia during the Croatian War of Independence. The operation began on 29 October 1991 and ended on 3 January 1992 when a nationwide ceasefire was signed to implement the Vance plan. The offensive was aimed at recapturing the region, in conjunction with two other HV offensives launched against SAO Western Slavonia in the north of the region within days.
On 14 November, the Navy blockade of Dalmatian ports was challenged by civilian ships. The confrontation culminated in the Battle of the Dalmatian channels, when Croatian coastal and island based artillery damaged, sank, or captured a number of Yugoslav navy vessels, including Mukos PČ 176, later rechristened PB 62 Šolta.