The Fourth Syrian War was one of a series of wars between Ptolemaic Egypt and the Seleucid Empire. After a period of historical confusion, we now enter the period covered by Polybius, so we have a better idea of the events of this war than of the earlier conflicts.
Antiochus III (The Great) had inherited his crown in 223 BC. The Seleucid Empire was in a poor shape in 223. In the east Parthia and Bactria-Sogdiana were both lost, one to nomad invaders, one to a Macedonia dynasty. Pergamum held most of Asia Minor. Even Seleuceia in Pieria, the port of Antioch-on-the-Orontes, his capital, was not in his hands, having been lost to Egypt during the Third Syrian War. His own replacement as governor of the eastern satraps, Molon, revolted in 222 and his general in Asia Minor, Achaeus, was soon to join in.
Antiochus decided to send his generals east to deal with Molon, while he launched an attack on the Egyptian positions in Coele-Syria. The campaign of 221 was a failure. Antiochus advanced south, but a Ptolemaic army under Theodotus of Aetolia blocked his progress at the southern end of the Marsyas valley. Meanwhile, in the east Molon defeated a Seleucid army under Xenoetas. The only potential bright spot for Antiochus was the death of Ptolemy III and his replacement by Ptolemy IV, a monarch with a much worse reputation.
In 220 Antiochus led an expedition against Molon. After crossing the Tigris, Antiochus caught up with Molon. Part of the rebel army defected once they realised that they were facing Antiochus in person. Faced by the collapse of his position, Molon killed himself. Many of his chief supporters followed suit. Having crushed one rebel, Antiochus now learnt that he faced another. While Antiochus was absent in the east, Achaeus rebelled in Asia Minor. However, this second rebellion quickly stalled, leaving Antiochus free to return to his war against Egypt, while Achaeus remained quiet.
The campaign of 219 began well, with the capture of Seleuceia in Pieria. Internal conflicts in the court of Ptolemy IV led Theodotus to change sides, taking with him the defensive line in Coele-Syria. Antiochus had a chance to advance into Egypt before any credible defence could be mounted, but he missed it. Another Aetolian general, Nicolaus, delayed him at the fortress of Dora (or Tantura), and Antiochus agreed to a series of negotiations that delayed him over the winter of 219-218.
Antiochus spent 218 slowly working his way through Coele-Syria, reducing a series of Ptolemaic strongholds. This was a war of sieges, with one major battle that came while Antiochus was working his way down the coast of Phoenicia. This came at the Plane Tree Pass between Sidon and Berytus, and saw Antiochus and his admiral Diognetus defeat a Ptolemaic army led by Nicolaus and supported by a fleet commanded by Perigenes. The year ended with Antiochus going into winter quarters at Ptolemais, on the coast south of Tyre.
While Antiochus was slowly working his way through Coele-Syria, Ptolemy’s chief minister Sosibius was creating a new army. Under Ptolemy III the Ptolemaic army had been allowed to decay, and even after Sosibius called troops back from Egypt’s overseas possessions and hired the best mercenaries they could find, the Egyptian army was still not large enough. Sosibius decided to arm native Egyptians.
This was a very dangerous precedent for the Ptolemaic regime – the last time Egyptians had been under arms had been almost a century in the past, at the battle of Gaza (312). 20,000 Egyptians were recruited and (probably) trained to fight in the phalanx, previously a Greek and Macedonian preserve. Their participation in this war seems to have greatly encouraged Egyptian nationalism, centred around the powerful priesthood.
The exact size of the Egyptian army is not clear. Polybius gives a figure of 70,000 men, although his detailed breakdown of the army could also support a figure of around 50,000 men. The Egyptian troops are referred to as either heavy armed or a phalanx and Polybius’s account can be read to give Ptolemy either 25,000 or 45,000 troops in the phalanx. Antiochus had 68,000 men, with a 20,000 strong phalanx, so regardless of the exact size of the Egyptian army, was outnumbered in the crucial heavy infantry.
The decisive battle came at Raphia, near Gaza, probably on 22 June 217. Both sides achieved success on their right wing, but while Antiochus took part in the pursuit on his right, Ptolemy extracted himself from the chaos, and led his phalanx to victory. Antiochus lost 10,000 dead and 4,000 captured, and was forced to retreat to Antioch. Once there he negotiated a peace treaty with Ptolemy in which he surrendered Coele-Syria, already lost to him, but kept Seleuceia in Pieria.
Ptolemy is sometimes represented as having missed a chance to greatly expand his possessions at the expense of Antiochus, but from an Egyptian point of view he had restored a defensive line in Syria that would last for the rest of his reign. Antiochus turned his attention to restoring his empire in Asia Minor and then the eastern satrapies.