In 118 AD Hadrian became emperor. He wanted more unity in the Roman Empire to get more stability. He traveled through the whole of his empire, and had buildings constructed in the places he visited, paid by the state, to demonstrate the generosity of Rome.
In 130 AD he came in Jerusalem. That had not recovered from the destruction of the city by Titus in the year 70, and therefore the emperor wanted to replace the city by a modern one, with modern buildings, including a temple of Jupiter.
Hadrian was used to get applause for his building plans, but he did not get it in Jerusalem. The Jews detested the idea: the few remains of the old Jerusalem would be destroyed as well, and the identity of Jewishm attacked.
Hadrian became angry with the Jews. He considered Jewism to be a backward religion, and their clinging to their traditions was contrary to his ideal of integration of all peoples to the culture of the Roman empire.
So he decided the Jews had to adapt. If they did not do this voluntary than violence would be used. Circumcision was forbidden (a barbarian ritual in his eyes), the study of the Torah was forbidden, as were public assemblies like the ones in synagogues.
The Jews concluded that a new revolt was inevitable.
This one was prepared thoroughly. A guerrilla-war like the one of Judas Maccabaeus had to be made again. It started in 132. Commander became Simon Ben Koseba, better known as Bar Kokhba, and Rabbi Akiva said he was the Messiah. The war lasted 3 years, but Bar Kokhba lost. Systematically each center of resistance was destroyed. According to Dio Cassius, a Roman author, 580.000 Jewish soldiers were killed, and a lot more civilians, because of famine and diseases. The number of Romans killed was comparably large, but that did not comfort the Jews. The survivors were banned from Judea and Jerusalem. Many went to Galilee, where they were allowed to live. Rabbi Akiva deserved respect by his death: while he was tortured horribly he continued to give explanations about the Torah.
Jerusalem's soil was ploughed and made flat with debris. It became a pure Roman city, with temples, markets, baths and and triumphal arches (for Hadrian). A temple for Venus was built on the place were Golgotha presumably was. Jerusalem also got a Roman name: Aelia Capitolinum, and Judea was called Palestina.
Attempts to destroy the Jewish faith and the Jews who adhered to it went on without hesitation. The Sanhedrin pronounced that Jews were allowed to pretend to denounce their faith in order to save their lives.
(Christians who were prosecuted were not supposed to do that).
This rule is still valid. When the Jews were prosecuted in Spain, from 1492 on, it was practised as well.
Had Hadrian lived for a long time the Jews might have been eradicated, but that did not happen: he died in 138, and was succeeded by Antoninus Pius, who made Jewism a legal religion again.
Hadrian has relatively damaged Jewism more than Adolf Hitler.
The most remarkable thing about that is the fact that historians did not blame Hadrian for attempting to eradicate Jewism, and still not do so. Hadrian still counts as one of the five good emperors of the second century.
With the removal of the Jews from Judea the Diaspora has really started.