In Antiquities of the Jews - Book XIV, Josephus tells the somewhat convoluted story of how Herod the Great managed to come to power in Judaea (Judea). The following is almost entirely taken from Josephus' account.
During the consulship of Quintus Hortensius and Quintus Metellus, during the third year of the 177th Olympiad (what we would normally call 69 B.C.), the oldest son of the ruler of Judaea began his high priesthood. This priest, called Hyrcanus, was the older brother of an ambitious man named Aristobulus. Hyrcanus had succeeded to the position of king/high priest when his mother Alexandra died, but Hyrcanus lacked ambition, and must have seemed cowardly to the leading families of Judaea. Even so, when his younger brother Aristobulus had provoked him sufficiently and challenged him, Hyrcanus did lead troops to Jericho to do battle against him. There, however, many of his troops deserted Hyrcanus, who wound up surrendering the power to his brother Aristobulus.
Not all the leading families liked the new arrangement. Among other friends of Hyrcanus was one Antipater of Idumea, who was rich and hated Aristobulus for what he had done to his friend. Antipater stirred up the leading Jewish families and by continually harping upon it to Hyrcanus, overcame the former high priest's reluctance to act.
Hyrcanus agreed to let Antipater go to the king of Arabia, Aretas, to ask his help in regaining his position as high priest/king. Having made a successful mission, Antipater returned to Jerusalem and secreted Hyrcanus out of the country to Petra, in Arabia. By promising to return the cities Hyrcanus' father Alexander had taken from Arabia (Medaba, Naballo, Libias, Tharabasa, Agala, Athone, Zoar, Orone, Marissa, Rudda, Lussa, and Oruba), Hyrcanus, with Antipater, persuaded King Aretas to help them regain the throne of Judaea.
With 50,000 horse and foot troops, Hyrcanus and Aretas led an expedition against Aristobulus and beat him. Aristobulus fled to the temple in Jerusalem. Aretas assaulted the temple and besieged Aristobulus.
Meanwhile Pompey had sent his brother-in-law, military tribune Marcus Aemilius Scaurus into Syria where the brothers sent ambassadors to plead their cases. Scaurus listened to ambassadors from both brothers, and sided with the younger brother. He ordered Aretas to leave Judaea or be declared an enemy of Rome.