What was the main reason behind the Anglo-French war?

The main reason behind the Anglo-French War was the dispute over colonial possessions in North America. This rivalry was further fueled by the prospect of trade in India as well. 

With the decline of Mughal power in the mid-18th century, the prospect of lucrative trade that would ultimately lead to a monopolistic hegemony was too tempting to pass up by either Britain and France.

For further reading check the following articles:

• Portuguese India
• British Raj
• Dutch Colonialism in India