In this Timeline, Antiochus IV Epiphanes of the Seleucid Empire succeeds in reforming the military of the Seleucid army after 166 B.C to resemble the Roman system. Having seen the complications of the phalanx wall - such as its lack of manoeuvrability and flexibility, the entire concept of the sarissa phalanx is replaced, bringing the Seleucid Empire greater military success, both against its old enemies such as the Ptolemaic dynasty and new ones like the Hayasdan and of course, the Roman Republic.
Context and Point of Divergence
Context: In the year 168 BCE, the Macedonian phalanx was utterly crushed by the Roman legions at Pydna. This was because as the Roman line withdrew, the phalanx pursued them on to rougher terrain, which created gaps in the phalanx. The Romans exploited this and won, ending the kingdom of Macedon.
PoD: In this timeline, Antiochus IV Epiphanes dies later than in OTL. He also reforms the Seleucid military along Roman lines, seeing the vulnerability of the phalanx.
168 BC: The kingdom of Macedon is turned into a Roman province due to the defeat of the phalanx.
166 BC: In the Daphne parade (as in OTL) 5000 Romanised troops are shown at the head of the army. In the following years, the phalanx is phased out and replaced with mobile heavy infantry wielding short broadswords, similar to the Gladius Hispaniensis.
164 BC: At the Battle of Beth Zur, thanks to the disciplined new Romanised troops, the ambush is repelled and the Seleucids win.
160 BC: The Maccabean Revolt has been defeated thanks to their guerrilla tactics being repelled. However, unrest is rife. Antiochus IV dies. His son, Antiochus V is crowned Emperor but at the age of 11.
157 BC: Simon Thassi leads another revolt against Seleucid control of Judaea. They are defeated, and to prevent further revolts, Antiochus V permits the worship of Judaism. He now turns his attention to the old enemy of the Seleucids - Ptolemaic Egypt.
155 BC: The Battle of Gaza. The Ptolemaic forces are smashed. Antiochus gains the lands north of the Red Sea.
147 BC: Following the fall of Alexandria, Egypt is annexed by Antiochus.
145 BC: The Roman consul, Lucius Hostilius Mancinus meets with Antiochus. Fearing their resurgence, he threatens invasion of the Seleucid Empire if the Seleucids do not withdraw from Egypt and also if there are any acts of aggression against Rome's allies in Anatolia. Antiochus refuses. It is war.