The Illyrian Wars were two wars fought mainly against piracy between Rome and Illyria (Croatia) between 229 and 219 B.C. They represent the first time the Romans crossed the Adriatic to protect trade routes.
Polybius says the war started following complaints by Italian merchants about piracy, which the Illyrian rulers. In 230 the Romans approached the Illyrian queen, Teuta, about the problem. Teuta seems to have permitted the execution of one of the Roman envoys. In contrast, Appian says the start of the war was an appeal to Rome by the Greeks living in Issa who were threatened by the Illyrian king Agron. In 229, Rome responded by sending both the consular armies and the 200 ships of the naval fleet it had built during the first Punic War. Teuta surrendered, gave up certain claims and paid reparations.
The Roman settlement with Illyria included granting certain Greek cities of Apollonia and Epidamnus, on the Adriatic coast, and the island of Corcyra (which the Illyrians had besieged) protection and the status of amicus 'friend'. This put Greek territory into Roman hands for the first time and made the fate of the cities its business. At this time, Greece was controlled primarily from Macedonia. Rome made an alliance with Macedonia.
In 219, Demetrius of Pharos (d. 214) used his island, off the Dalmatian coast, as the homebase for piracy and attacked territories Rome now protected. The two consular armies once more crossed the Adriatic, and besieged the Illyrian cities of Pharos and Dinale. So Demetrius fled to Philip of Macedon. The Romans took Pharos in a day; Dinale, in seven; and again made a settlement reinstating the terms of the earlier agreement, and favoring Illyrians who opposed Macedonians.
"Kleemporos," by P. S. Derow; Phoenix Vol. 27, No. 2 (Summer, 1973), pp. 118-134, Cary and Scullard in A History of Rome, Ground warfare: an international encyclopedia, Volume 1, by Stanley Sandler, Dictionary of Wars, by George C. Kohn, and Roman History, by Julius Koch J. M. Dent & co., 1901.
Appian 7.17-22 and Polybius 2.2-8 (according to Derow) are the main ancient sources on the first Ilyrian War.