By the end of the 16th century, the Ottomans occupied a considerable territory on the Croatian battlefield and pushed the line of defense to the Senj-Bihać-Sisak line. Only the central and northern part of the kingdom remained free, thus reducing Croatia to, as it was called in Latin, “reliquiae reliquiarum” or “remnants of remnants”.
Sisak is located 100 km from Zagreb to the east at the confluence of the river Kupa and Sava. In 1550, the archdiocese of Zagreb built a fortress there to protect the crossing of the river. The fortress was built in the renaissance style in the shape of a triangle with cannon towers in each corner. It had extremely solid walls that were difficult to break through. It was also surrounded by a moat with only one entrance to which a small bridge led. There were many loopholes on the walls and towers from which one could shoot in all directions. There were 300 soldiers in it.
In 1590, the Ottoman sultan Murat III made peace with Persia and turned to war in Europe. In 1593, the Bosnian Hasan pasha Predojević with about 12,000 soldiers (mostly spahis) began to invade Croatian lands, ravaging everything he could. He surrounded Bihać, which was the most advanced point of the defense. Although the defenders begged for help, the city soon fell. Predojević then turned towards Sisak and occupied some smaller fortifications along the way. On June 15, 1593. the Turks came to Sisak where they set up a large camp and started to think about a strategy of an attack. Already after the fall of Bihać, the Croatian Sabor (parliament) passed an insurrection (insurrectio), a decision on a general uprising, where it was written that at the ban’s invitation nobles, serfs, citizens and all clergymen must go to war. If Sisak fell to the Ottomans, the road to Zagreb would be open, as well as to the Habsburg hereditary estates of Carniola, Styria and Carinthia (modern Slovenia and southern Austria). The Christian army that gathered numbered about 6,000 people (mostly Croats and some from the Slovenian and Austrian provinces), and was composed mostly of professional soldiers. There were about 1,000 cavalry (heavy and light cavalry), 4,000 well-equipped and trained armored infantry, and about 1,000 arquebusiers and archers. The army was commanded by Croatian ban (king’s viceroy) Toma Bakač Erdody as the leader of the Croatian part of the army, duke Ruprecht Eggenberg who commanded the Austrian units and Andrija Auersperg as the leader of the Slovenian ones.
After arriving in front of Sisak, the Ottomans beat the cannons towards the fort and asked the crew to surrender, but the crew retaliated in the same way, keeping them at a safe distance. On June 22, at about 10 am, Christian forces finally arrived. Meanwhile, the Turks crossed a small bridge to the northern part of the Kupa river, leaving the bridge and the large river behind. The first attack was launched by Christian cavalry. Together, they struck extremely hard at the turkish ranks. In a hard battle after some time the janissaries repulsed the attack and forced the christians to retreat. The Ottoman cavalry counterattacked, but then the arquebusiers and archers fired. They were pelted with a pile of bullets and arrows that were hiting them easily. The commotion and insecurity in the turkish ranks began, and then under the sound of trumpets and with flags raised high the christian cavalry turned and struck again at the Turks. The heavily armored infantry also approached. At this moment the bulk of the Turkish army was routed and they began to flee towards the river. Seeing what was happening, the Sisak crew came out of the fort and in a small breakthrough captured and blocked the bridge with weapons. The Ottoman army suddenly found itself completely surrounded and disoriented and in a panic began to jump into the river. The vast majority of them drowned, and the rest were cut down by christian troops. About 8,000 Turks lost their lives in the battle, including Hasan pasha Predojević, his brother and many other Ottoman dignitaries. Also was captured a large amount of military equipment (cannons, rifles, ammunition) as well as everything else that was in their camp. Losses on the christian side were minimal.
The Battle of Sisak was a turning point in the wars with the Ottomans on Croatian territory. For the first time since their arrival, a balance of power was established on the battlefield and Turkish progress was halted. Also the war slowly shifted from a defensive one to offensive one. This battle also marked a change in the balance of power in the rest of Europe and a gradual weakening of Ottoman power. A printed leaflet soon appeared in Europe showing the first great victory of the united Christian forces in central Europe against the Turks. The commanders of the army were praised throughout Europe. Among them, by pope Clement VIII, emperor of the Holy Roman Empire Rudolf II. and king of Spain Philip II.