Mir Jafar betrayed Siraj-ud-Daula because Lord Clive had told Mir Jafar previously that if he would help Lord Clive, he will be given a lavish amount of money, prestigious position and joyful life. With the help of the Nawab's uncle, Mir Jaffar, and local moneylenders, the Jagat Seths, Siraj ud-Daulah was betrayed. After that Clive defeated Siraj-ud-Daulah at Plassey in 1757 and captured Calcutta. The battle was preceded by an attack on British-controlled Calcutta by Nawab Siraj-ud-Daulah and the Black Hole massacre.
This Question is Also Available in:
हिन्दी