De Europa Antigonidorum, Book 4, Chapter 14, by Lucius Haemonus:
There had ben strange tidings from the north and change was coming. King Cassander had always been careful of the tribal developments in the north and had taken care to ensure no tribe got too powerful, but Antigonus II had other matters to attend to, namely the political web of managing the Hellenic Leagues. Thus, he made no moves against the new arrivals in the Balkan. Even if he had time, though, he cared little for the tribes of the north.
The Boii and Volcae had already begun moving into Illyria and Pannonia during the life of Alexander, and had conquered and subjugated many tribes. The first Balkan tribe to be defeated were the Illyrian Autariatae, who had been the most powerful. With such a victory they quickly made themselves known throughout the Balkan and they moved further south and south, subjugating more and more with their great army. Hundreds of thousands moved south, quickly spreading Celtic culture and quickly completing the Celtization of the Balkan. In the years during the Diadochi Wars, the Celtic general Molistomos had completed the subjugation of Illyria, conquering the lands of the the Triballi, Dardanian and Paeonian tribes. Their military power was frightening and the Illyrians were thus no match against their power.
Many of them clamored for an attack on Macedonia and Thrace after these victories, but their leaders were too afraid of the Macedonians. Perhaps they would have fought against Cassander, but they would not fight so soon against the empire that had conquered Macedonia and fully controlled Greece. An attack on Macedonia had been dismissed as an option, though probing warbands raided Macedonia often enough that Antigonus was forced to spend thousands of men watching the borders. No, the Boii and Volcae had different ambitions. They watched as the kingdom of Lysimachus collapsed and gathered their armies. Seutes had made himself king of northern Thrace, but he was no undisputed master and lord. A great army under the leadership of the general Cambaules marched on Thrace and the kingdom of Seutes. Seutes had asked Antigonus for help, who had sent a contingent of Macedonians, but not nearly enough. Near the Danube Seutes and Cambaules met in battle and though the Thracians fought valiantly, Cambaules defeated them, for he possessed a greater army. The Macedonians were ransomed to Antigonus again. After that, Cambaules drove Thracians into the south, and subjugated the tribes of Thrace, completely conquering that land. Many Thracian tribes moved into Antigonid Thrace. After this great victory, he moved north into the lands of the Getae, who he subjugated. Finally, he pushed through Dacia, linking up with the Celts that lived there. After that, the great general Cambaules moved to Tylis and made it his capital.
The Boii and Volcae thus won many victories and conquered the Balkans, but still they did not dare press into Macedonia. Rather, they settled in the Balkans, establishing their kingdoms. In the south, Antigonus was finally worried due to the threat of the Celts and used many men to stop the marauding warbands. Nonetheless, his attentions were elsewhere, for the Thracian tribes in Antigonid Thrace were marauding and besieging. Antigonus marched with an army of 30,000, and offered settlement in his lands, but only if they would submit and send troops to aid him. Many Thracians accepted this offer, though a few tribes had to be defeated in battle. This happened near Lysimachia, where Antigonus defeated a contingent of 15,000 Thracians. They proved to be little match for the phalanx and Thessalian cavalry, and thus surrendered quickly. As punishment, these Thracians were sent to Armenia, where they still live, according to legends.
After this, Antigonus once more focused on the issues his father had worried about. Ptolemy had established himself in Sicily and had begun expanding into southern Italy. Antigonus looked at these developments with a sour face and offered his protection to the cities of Southern Italy, as well as membership of the Hellenic League. Croton, Scylletium and Terina, the cities most threatened by Ptolemy, accepted. The cities to the south of them had already been conquered by the new king of Sicily. However, Ptolemy II took little heed of this, as Antigonus had hoped. He marched on Scylletium and besieged it. Antigonus, then, put together an expedition of 10,000 Macedonians. He had not only gone against Antigonus, but also against the entire Hellenic League by besieging one of their member cities. Perhaps Ptolemy thought the Hellenic League was toothless, but he would soon be proven wrong. The representatives of Scylletium pleaded before the assembly of the Hellenic League and told them of the evil tyrant Ptolemy, who would stop at nothing but complete subjugation and had already conquered Sicily. T Enraged, the Greek cities raised a force of 20,000 on their own. he last trick of Antigonus was to enlist the help of Epirus, offering money, and thus Pyrrhus raised 10,000 men of his own. With this, Antigonus had created a pan-Hellenic front, from the Greeks to the south, to the Macedonians of the north and the ''barbarian'' Epirotes. Thus Antigonus II sent his general, Nicanor, to set sail to southern Italy, with a force of 40,000 men, with Pyrrhus. He stayed behind in Macedonia, watching the Celts with unease.
Note: The not invading Macedonia in this instance isn't the famed Brennus expedition, but the expedition in 298 BC where Cassander beat them at Mount Haemus.