Reason for disunity of Indians during …

There are a good number of reasons why the British were able to do so, and in fact rule over India effectively for over a century.

• India was more a collection of warring princely states, at loggerheads with each other. The British successfully used this to play off one state against another. Add to it there was no dearth of people willing to betray the kingdom for a few pieces of silver. Robert Clive succeeded at Plassey, because Mir Jaffar was willing to betray his master Siraj-Ud-Daulah in lie of being the Nawab. Mir Jaffar himself was betrayed by Mir Qasim later on.Tipu Sultan one of the most redoubtable fighters against the British rule, was finally defeated, as the Marathas, King of Mysore, Nizam of Hyderabad all joined hands with the British.
• The other colonial powers in India competing for the share of resources were France,Portugal,Denmark, Holland. Of the 4, Denmark and Holland could never really be serious competitors to the British, they had their own trading posts, scattered around, but were never a serious threat. Portugal focussed primarily on the Western coast, Goa, parts of Kerala, Karnataka, and this left the British with vast swathes of unoccupied territory. That left France as the major contender to Britian in the race for colonialism. The British Army was more well equipped, more professional, more disciplined compared to the French army, suffering from indiscipline and corruption. This made the British win key battles all over the East Coast, as they effectively grabbed control.