Clive was anxious to start a war against Sirajuddaula but he was in search of a proper reasons for it he wrote a letter to Nawab about the violation of the terms of the treaty of Alinagar and without waiting for the reply of the letter he sent an army against Murshidabad.
Nawab also sent his army to face the invasion of the English and a pitched battle was fought between the two dries in the field of Plessey on 23rd June 1757 A. D.
The following reasons contributed to the outbreak of this significant war:
(a) The English wanted to establish their political, administrative and economic supremacy over Bengal for which it was essential to wage war against the Nawab. So they wanted to dethrone the Nawab, Sirajuddaula and denthroned Mir Jafar their puppet in his place and fought the decisive battle of Plessey.
(b) The relatives of Sirajuddaula opposed him. The English supported Shaukat Jang but they failed to remove Sirajuddaula. As Sirajuddaula began to watch their activities very minutely, they decided to remove him from the throne by all means.
(c) The English never respected the Nawab, hence the feeling of hatred also developed in the heart of the Nawab for the English. However, the Nawab wanted to strike a compromise with them but he failed and war broke out between the two.
(d) The British knew that in spite of all power and efficiency, the army of Sirajuddaula would prove to be worthless because of the traitors. The English concluded a secret treaty with Mir Jafar on 4thJune 1757. In enhanced the power of the English. However, the army of Sirajuddaula was strong enough to defeat the English but the teacher of Mir Jaffar mined him.
(e) Sirajuddaula thought the French to be more trust worthy than the English. He was also sad at the loss of Chandanagar and his defeat at the hands of the English.
The English pressurised the Nawab to outset and French from India and not to allow them to continue their trade in India. Moreover, some Frenchmen were serving in the army of the Nawab, so he could not out all the French from Bengal. Hence, the war was inevitable between the two.
(f) Clive had twice defeated the Indian army and he considered the Indian army no more than a mob which could be terrified by the sound of the bullet. As Clive had joined hands with Mir Jafar, the Commander of Sirajuddaula. Durlabh Rai and some other military personnel, he felt his position to be quite strong and so could think of making an invasion against Sirajuddaula who was surrounded by traitors.