Thus the winter passed away. With the spring of 555 the proconsul Publius Sulpicius broke up from his winter camp, determined to conduct his legions from Apollonia by the shortest route into Macedonia proper. This principal attack from the west was to be supported by three subordinate attacks; on the north by an invasion of the Dardani and Illyrians; on the east by an attack on the part of the combined fleet of the Romans and allies, which assembled at Aegina; while lastly the Athamanes, and the Aetolians also, if the attempt to induce them to share in the struggle should prove successful, were to advance from the south. After Galba had crossed the mountains pierced by the Apsus (now the Beratind), and had marched through the fertile plain of Dassaretia, he reached the mountain range which separates Illyria from Macedonia, and crossing it, entered the proper Macedonian territory. Philip had marched to meet him; but in the extensive and thinly-peopled regions of Macedonia the antagonists for a time sought each other in vain; at length they met in the province of Lyncestis, a fertile but marshy plain not far from the north-western frontier, and encamped not 1000 paces apart. Philip's army, after he had been joined by the corps detached to occupy the northern passes, numbered about 20,000 infantry and 2000 cavalry; the Roman army was nearly as strong. The Macedonians however had the great advantage, that, fighting in their native land and well acquainted with its highways and byways, they had little trouble in procuring supplies of provisions, while they had encamped so close to the Romans that the latter could not venture to disperse for any extensive foraging. The consul repeatedly offered battle, but the king persisted in declining it; and the combats between the light troops, although the Romans gained some advantages in them, produced no material alteration. Galba was obliged to break up his camp and to pitch another eight miles off at Octolophus, where he conceived that he could more easily procure supplies. But here too the divisions sent out were destroyed by the light troops and cavalry of the Macedonians; the legions were obliged to come to their help, whereupon the Macedonian vanguard, which had advanced too far, were driven back to their camp with heavy loss; the king himself lost his horse in the action, and only saved his life through the magnanimous self-devotion of one of his troopers. From this perilous position the Romans were liberated through the better success of the subordinate attacks which Galba had directed the allies to make, or rather through the weakness of the Macedonian forces. Although Philip had instituted levies as large as possible in his own dominions, and had enlisted Roman deserters and other mercenaries, he had not been able to bring into the field (over and above the garrisons in Asia Minor and Thrace) more than the army, with which in person he confronted the consul; and besides, in order to form even this, he had been obliged to leave the northern passes in the Pelagonian territory undefended. For the protection of the east coast he relied partly on the orders which he had given for the laying waste of the islands of Sciathus and Peparethus, which might have furnished a station to the enemy's fleet, partly on the garrisoning of Thasos and the coast and on the fleet organized at Demetrias under Heraclides. For the south frontier be had been obliged to reckon solely upon the more than doubtful neutrality of the Aetolians. These now suddenly joined the league against Macedonia, and immediately in conjunction with the Athamanes penetrated into Thessaly, while simultaneously the Dardani and Illyrians overran the northern provinces, and the Roman fleet under Lucius Apustius, departing from Corcyra, appeared in the eastern waters, where the ships of Attalus, the Rhodians, and the Istrians joined it.
Philip, on learning this, voluntarily abandoned his position and retreated in an easterly direction: whether he did so in order to repel the probably unexpected invasion of the Aetolians, or to draw the Roman army after him with a view to its destruction, or to take either of these courses according to circumstances, cannot well be determined. He managed his retreat so dexterously that Galba, who adopted the rash resolution of following him, lost his track, and Philip was enabled to reach by a flank movement, and to occupy, the narrow pass which separates the provinces of Lyncestis and Eordaea, with the view of awaiting the Romans and giving them a warm reception there. A battle took place on the spot which he had selected; but the long Macedonian spears proved unserviceable on the wooded and uneven ground. The Macedonians were partly turned, partly broken, and lost many men.
But, although Philip's army was after this unfortunate action no longer able to prevent the advance of the Romans, the latter were themselves afraid to encounter further unknown dangers in an impassable and hostile country; and returned to Apollonia, after they had laid waste the fertile provinces of Upper Macedonia - Eordaea, Elymaea, and Orestis. Celetrum, the most considerable town of Orestis (now Kastoria, on a peninsula in the lake of the same name), had surrendered to them: it was the only Macedonian town that opened its gates to the Romans. In the Illyrian land Pelium, the city of the Dassaretae, on the upper confluents of the Apsus, was taken by storm and strongly garrisoned to serve as a future basis for a similar expedition.
Philip did not disturb the Roman main army in its retreat, but turned by forced marches against the Aetolians and Athamanians who, in the belief that the legions were occupying the attention of the king, were fearlessly and recklessly plundering the rich vale of the Peneius, defeated them completely, and compelled such as did not fall to make their escape singly through the well-known mountain paths. The effective strength of the confederacy was not a little diminished by this defeat, and not less by the numerous enlistments made in Aetolia on Egyptian account. The Dardani were chased back over the mountains by Athena-goras, the leader of Philip's light troops, without difficulty and with severe loss. The Roman fleet also did not accomplish much; it expelled the Macedonian garrison from Andros, punished Euboea and Sciathus, and then made attempts on the Chalcidian peninsula, which were, however, vigorously repulsed by the Macedonian garrison at Mende. The rest of the summer was spent in the capture of Oreus in Euboea, which was long delayed by the resolute defence of the Macedonian garrison. The weak Macedonian fleet under Heraclides remained inactive at Heraclea, and did not venture to dispute the possession of the sea with the enemy. The latter went early to winter quarters, the Romans proceeding to the Piraeeus and Corcyra, the Rhodians and Pergamenes going home.
Philip might on the whole congratulate himself upon the results of this campaign. The Roman troops, after an extremely troublesome campaign, stood in autumn precisely on the spot whence they had started in spring; and, but for the well-timed interposition of the Aetolians and the unexpected success of the battle at the pass of Eordaea, perhaps not a man of their entire force would have again seen the Roman territory. The fourfold offensive had everywhere failed in its object, and not only did Philip in autumn see his whole dominions cleared of the enemy, but he was able to make an attempt - which, however, miscarried - to wrest from the Aetolians the strong town of Thaumaci, situated on the Aetolo-Thessalian frontier and commanding the plain of the Peneius. If Antiochus, for whose coming Philip vainly supplicated the gods, should unite with him in the next campaign, he might anticipate great successes. For a moment it seemed as if Antiochus was disposed to do so; his army appeared in Asia Minor, and occupied some townships of king Attalus, who requested military protection from the Romans. The latter, however, were not anxious to urge the great-king at this time to a breach: they sent envoys, who in fact obtained an evacuation of the dominions of Attalus. From that quarter Philip had nothing to hope for.