Indians has no sense of nation, of being one people until British colonialization of India.
Take a look at this Battles fought in 17th and 18th century-
1757- Battle of Plassey- war between siraj-ud-daulah ( nawab of Bengal) and East India Company (EIC) helped by Mir Jafar.
1766-1st Anglo Mysore war- between Haider Ali( Mysore) and EIC helped by Nizam of Hyderabad.
1775- 1st Anglo Maratha war- between madav Rao II peshawe(pune) and EIC helped by Raghunath Rao.
1782-2nd Anglo Mysore war- between Haider Ali and EIC helped by Nizam and Marathas.
The most decisive battle, which changed the fate of India , was 3rd battle of Panipat- fought between Marathas and Ahmad shah Abdali( Afghan ruler) supported by Najib khan(Rohilakhand) and shuja-ud-daulah( Awadh) in 1761. The maratha army was completely routed. Their defeat gave an opportunity to English EIC to consolidate its power in Bengal and South India. In fact, the third battle of Panipat did not decide who was to rule India but rather who was not. The way was, therefore, cleared for rise of British power in India.
If there is feeling if nationalism among the people of India, then one state of India would have helped other Indian state against any foreigner enemy. But the bitter truth here was that, not only they didn't helped their native state but they also joined hands with the foreigners against other Indian state for their personal greed.
Well upto 1857, there was absence of spirit of political nationalism among the people of India. This was because India at that time lacked the elements which constitute modern nation. The people of India did not feel they were all Indians, nor they conscious of oneness, even though elements of cultural unity had existed in country for centuries. Therefore, there did not exist the ideal living and dying for one's nation. Instead people were loyal to persons, tribes, castes and religious sector.