In 49 BCE, Julius Caesar was pursuing Pompey all the way to Egypt.
"Once there, he committed a rare tactical blunder and found himself besieged in Alexandria by Pompey’s army and its allies. Sorely in need of friends, he looked for any help that would extricate him from his dangerous situation.
Until that time, Hyrcanus had been an official ally of Pompey. However, he shrewdly switched sides and declared his allegiance to Caesar. He then committed over 3,000 Jewish soldiers to an expeditionary force that invaded Egypt and helped raise the siege of Alexandria.
Thus, when the Roman civil war ended in Julius Caesar’s complete victory Hyrcanus was in a fortuitous position. Indeed, Caesar showed the Jews his gratitude for their help. He revoked the harsh decrees and burdensome taxation imposed by Pompey. He also allowed the walls and fortifications of Jerusalem to be rebuilt and restored Jaffa as well as a number of other coastal cities to Jewish rule."
The assassination of Caesar in 44 BCE worried the Jews: "Would his successor be as positively disposed toward them? Tragically, that eventual successor, Marc Antony, gave power to a man whose rule was as antithetical to Jewish principles and ideals as imaginable."
The man was Herod, the murder of his sons.