Antiochus V Eupator (Greek: Αντίοχος Ε' Ευπάτωρ, ca. 172 BC - 161 BC), was a ruler of the Greek Seleucid Empire who reigned 163-161 BC. The dates are from 1 Maccabees 6:16 and 7:1, in A.S.149 (312-149 = 163 B.C.) and A.S.151 (312-151 = 161 B.C.). The calculation assumes that 1 Maccabees used the Babyloian reckoning (Nisan-years: beginning from the day after the New Moon nearest to the Spring Equinox) for both Jewish sourced dates and Syrian sources dates, not using the Macedonian-years (beginning from the day after the New Moon after the Autum Equinox) for the latter. The Seleucid Era (Anno Seleucidarum) began from Oct/Nov 312 B.C., when Seleucus I took back Babylon.
Antiochus V was only nine when he succeeded to the kingship, following the death in Persia of his father Antiochus IV Epiphanes and Laodice IV. Regent for the boy was the general Lysias who had been left in charge of Syria by Epiphanes. Lysias was, however, seriously challenged by other generals and was therefore in a precarious situation. The Roman Senate still kept as hostage Demetrius, the son of Seleucus IV and the rightful heir to the throne, age twenty two then. The Senate refused to release Demetrius, because they considered that it is better to have Syria rulled by a boy than a man. 
Though still a boy, Antiochus V was a strong willed king. He attemptted to quell the Maccabean Revolt in Judea, but ended in a weak compromise. He at first had a military victory in the Battle of Beth-Zecharia, killing Eleazar, a brother of Judas Maccabaeus, for the still very fearsome Seleucid army , and the fact that 163 B.C. was a sabbatical year in the second-temple-period cycle, thus the Jews had not enough provisions to withstand a siege.  When Lysias heard that Philip, a confidant of Antiochus IV Epiphanes, who went with the previous king to conquer Mesopotamia, and was entrusted to bring up Antiochus V before the king's death, came back to the capital with the other half of the Seleucid army, Lysias felt threatened, and advised Antiochus V to offer peace to the Jews. The Jews accepted, but the king broke his promise and tore down the walls of Jerusakem before he left. Lysias and Antiochus V found Philip in control of the capital Antiochus, but they defeated Philip and retook Antioch. 
When the Roman senate heard that the Syrian kept more warsips and elephants than allowed by the peace treaty of Apamea made in 188 B.C., they sent a Roman embassy to travell along the cities of Syria and crippled Seleucid military power. Warships were sunk and elephants hamstrung. Lysias dared do nothing to oppose the Romans, but his subservience so enraged his Syrian subjects that the Roman envoy Gnaeus Octavius (consul of 165 BC) was assassinated in Laodicea (162 BC). 
At this juncture Demetrius escaped from Rome and was received in Syria as the true king. Antiochus Eupator (Greek: Ευπάτωρ, whose epithet means "of a good father") was soon put to death together with his protector Lysias.