Successes in a foreign war are necessary if Octavian is to compete with Antony's military seniority; thus, Octavian in 35 initiates the Roman conquest of Pannonia, an area corresponding to present western Hungary and parts of eastern Austria, Slovenia, and northern Yugoslavia (Vojvodina).

Octavian conquers the coastal region and seizes inland Celtic and Illyrian strongholds, fighting three successive campaigns in Illyricum and Dalmatia (parts of modern Slovenia and Croatia) in order to protect the northeastern approaches of Italy.

In these, Agrippa has a distinguished military role.

Octavian eventually conquers and occupies Siscia (Sisak) in present Croatia.

Augustus had in 35 BCE conquered and occupied Segestica (later Siscia), the central city of Pannonia, now home to various Celtic tribes.

The country is not, however, definitely subdued until 9 BCE, when it is incorporated into Illyricum, the frontier of which thus extends as far as the Danube.

Pannonia Superior is divided into Pannonia Prima and Pannonia Ripariensis (or Savia).

…the provinces of Pannonia and Moesia.

The usurper Magnentius, refusing Constantius' offer of compromise and suffering setbacks at Siscia (modern Sisak, Croatia) and Sirmium (modern Sremska Mitrovica, Serbia), pursues Constantius to …

Maximus, upon the defeat of his troops near Siscia and at Poetovio, in Illyricum (in the Balkans) in July, flees to Italy, pursued by Theodosius as his navy gains command of the Adriatic Sea.

The many tribes—Danes, Obotrites, Slovenes, Bretons, Basques—that inhabit the frontier lands at the start of the reign of Louis the German over Eastern Francia had remained in awe at the power of the Frankish emperor and dared not stir up any trouble.

The Sorbs in 818 had rebelled, however, and had been quickly followed by Slavomir, chief of the Obotrites, who had been captured and abandoned by his own people, being replaced in 818 by Ceadrag.

Soon, Ceadrag too had turned against the Franks and allied with the Danes, who will shortly become the greatest menace of the Franks.

A greater Slavic menace is gathering on the southeast, where, Ljudevit Posavski, the Croat Prince of Savia from about 810, is harassing the border at the Drava and Sava rivers.

As the ruler of the Pannonian Croats, his successful resistance of Frankish domination is an important precursor to the eventual unification of Croatian duchies in Dalmatia and Pannonia into the Kingdom of Croatia.

The margrave of Friuli, Cadolah, had been sent out against him, but he had died on campaign and in 820 his margarvate had been invaded by Slovenes.

An alliance had been made in 821 with Borna, duke of Dalmatia, and in 822 Ljudevit is brought to heel.

After being forced to flee, Ljudevit becomes a ruler over the Serbs, maintaining close ties with the Carantanians and Timokians (the Slavs, or Vlachs, in the Timok River valley).

Peace continues in the Frankish realms until 827, when the younger Louis, who had received Bavaria and the neighboring marches, the former realm of Lothair, has to deal with a Bulgarian horde descending on Pannonia.

Tough Imperial troops fresh from savage fighting in the Low Countries have come under the supreme command of the Styrian general Ruprecht von Eggenberg.

Their crushing defeat of the Ottoman troops of Bosnian governor-general, or Beylerbeyi, Hasan Pasha Predojević at Sissek (Sisak) on June 22, 1593, enlarges the hostilities into a full-scale war.

Croatian troops are led by the Ban of Croatia, Tamás Erdődy, and some forces from the Duchy of Carniola are led by Andreas von Auersperg (Slovene: Andrej Turjaški), nicknamed the "Carniolian Achilles".

Recent analysis of several hitherto unknown or unused Ottoman sources has shown that there seems to have been a conflict of interest between the policy of the central Ottoman administration and the aims of the belligerent Hasan Pasha.

It appears that the struggle for more land and power was an important incentive for the offensive action on the side of the Bosnian sipahis, an action which at this time is not really in accordance with Constantinople’s policy.

The Sultan, on the other hand, may have felt that such an embarrassing defeat even of a vassal acting independently could not go unavenged if he himself was not to lose face.

The Battle of Sisak is the prelude to the long Second Ottoman-Habsburg War that begins in earnest on July 29, 1593, when the Ottoman army under Sinan Pasha, the Grand Vizier, launches a military campaign against the Habsburg monarchy, marshaling a large army of thirteen thousand Janissaries plus numerous European levies against the Christians.