Ant., XIV, 127 ff.; BJ, I, 187 ff.
There were a number of reasons for the sorrow evinced by the Jews of Rome at the assassination of Julius Caesar. He had been the enemy of Pompey, who had put an end to Jewish independence, sacrilegiously entered the Holy of Holies in the Temple and curtailed the territory of Judaea. Hence from the outbreak of the civil war Caesar considered the Jews his natural allies. He even intended to put the former Jewish King, Aristobulus II, in command of Roman forces against Pompey in Judaea. The plan was not implemented because Aristobulus died by poison; cf. Ant., XIV, 123 f.; BJ, I, 183 f.; see also Cassius Dio, XLI, 18:1(No. 409).
After his victory in the civil war, Caesar promulgated a series of edicts on behalf of the Jews in Judaea, improving their political situation. These abolished the enactments of Gabinius unfavourable to the interests of Judaea, returned to Judah the port of Jaffa, and increased the prestige of the high priest Hyrcanus; cf. Ant., XIV, 190 ff.
The Jews living in Rome itself were also favourably treated by the dictator. Thus, while Caesar enacted legislation aimed at the abolition of the various collegia (cf. Divus Iulius, 42:3: “cuncta collegia praeter antiquitus constituta distraxit”), the Jews were included by him among the collegia that had their rights of association confirmed. The sentence is a direct quotation from a letter of Caesar Augustus to Tiberius:
The Jews on their side gave proof of their friendship to Caesar by the aid they offered him when he was in straits in Alexandria in 47 B.C.E. Soldiers sent by the government of Judaea took part in the expeditionary force led by Mithridates of Pergamum (e.g. Turkey) to help Caesar, and the Egyptian Jews who guarded the entrances of the country opened them to the army of Mithridates, having been persuaded to do so by the letters of Hyrcanus, the Jewish high priest and ethnarch in Jerusalem.
Source: Divus Iulius, 84:5, in: Stern 1980, 109.; Dr. Joan Goodnick Westenholz. The Jewish Presence in Ancient Rome (p. 67 – 68)