Why did India went to war?

i think that india and pakistan where one part and becouse the pakistanians where wanted that the india people to leave started war and so there was a war of the sikhs(india) and the pakistanians(muslims) and then the sikhs went seperated from pakistanians and became country india (where the sikhs wen to) I guess or something like that look for the movie partition its about that kind of. I hope you understand my explanation :s