Judaean campaign
Following the defeat of Alexander Jannaeus at the hands of the Nabataeans, Judaea was caught in a civil war between the king and a religious group called the Pharisees. According to Josephus, Alexander Jannaeus' opponents persuaded Demetrius III to invade Judaea as it would be conquered easily owing to the civil war. Josephus gave two accounts regarding the numerical strength of Demetrius III; in Antiquities of the Jews, the Syrian king had 3,000 cavalry and 40,000 infantry. In the Jewish War, Demetrius III commanded 3,000 cavalry and 14,000 infantry. The latter number is more logical; the number given in Antiquities of the Jews could be an error by a copyist.
The date of the campaign is unclear in Josephus's account. 88 BC is traditionally considered the date of Demetrius III's Judaean campaign, but numismatic evidence shows that coin production increased massively in Damascus in 222 SE (91/90 BC) and 223 SE (90/89 BC). This increase indicates that Demetrius was securing the necessary funds for his campaign, making 89 BC more likely as the date of the invasion. The political situation in Syria in 89 BC helped Demetrius III initiate his invasion of Judaea; Antiochus X was in Antioch while Damascus was firmly in the hands of Demetrius III and there is no indication of a war with his brother Philip I.
The motives of Demetrius III are not specified in Josephus's account; the historian gives the impression that Demetrius III helped the Pharisees free of charge, which is hard to accept. The Syrian king's help must have been conditioned on political concessions by the Jewish rebels. Evidence for Demetrius III's motives is provided by the Pesher Nahum, which reads: [Interpreted, this concerns Deme]trius king of Greece who sought, on the counsel of those who seek smooth things, to enter Jerusalem; the academic consensus identifies "Demetrius king of Greece" with Demetrius III. The motive leading Demetrius III to attack Judaea might not have had anything to do with the call of the Pharisees. If Alexander Jannaeus took advantage of Demetrius's absence in 93 BC to wrest control of Damascus, then the invasion was probably in retribution against Judaea. It is also possible that Demetrius wanted to take advantage of Judaea's human resources in the struggle against his rivals to the Syrian throne. Finally, the Syrian kings, including Demetrius III's father, never fully accepted the independence of Judaea and entertained plans to reconquer it; the campaign of Demetrius III can be seen in this context.
Demetrius III was the first Seleucid king to set foot in Judaea since Antiochus VII (died 129 BC); according to Josephus, the Syrian king came with his army to the vicinity of Shechem (near Nablus), which he chose as the site for his camp. Alexander Jannaeus marched to meet his enemy; Demetrius III tried to persuade Alexander Jannaeus's mercenaries to defect as they were Greeks like him, but the troops did not answer his call. Following this failed attempt, the two kings engaged in battle; Demetrius III lost many troops but decimated Alexander Jannaeus's mercenaries and gained victory. The Judaean king fled to the nearby mountains and according to Josephus, when the 6,000 Judaean rebels in Demetrius III's ranks saw this, they felt pity for their king and deserted Demetrius to join Alexander Jannaeus. At this point, Demetrius III withdrew back to Syria.
The account of Josephus indicates that Demetrius III ended his Judaean campaign because his Jewish allies deserted him, but this is hard to accept; Josephus probably inflated the numbers of Judaeans in the Syrian army, and Demetrius III still had enough soldiers to wage wars in Syria after he departed Judaea. It is more likely that events in Syria forced Demetrius III to conclude his invasion of Judaea. Probably in 88 BC, Antiochus X died while fighting the Parthians, and this must have forced Demetrius III to rush north and fill the power vacuum before Philip I. Demetrius III may have feared that his brother would turn on him and try to take Damascus for himself; if it was not for the death of Antiochus X, Demetrius III would probably have conquered Judaea.