How was Hyderabad forced to join India after partition in 1947?

This report was submitted in 1949 but the subsequent governments suppressed its dissemination until it was declassified after Historian Sunil Purushotham from the University of Cambridge filed a petition to obtain a copy. "Soon after the British agreed to leave India and its subsequent partition in 1947, more than 500 princely states that had autonomy under British rule had acceded to either of the two countries - India or Pakistan. However, the Nizam of Hyderabad chose to remain independent and refused to join the Indian union. The Indian Government launched a military operation on 13th September 1948 called, "Operation Polo", resulting in the successful annexation of Hyderabad into the Indian union.

17th September 1948 - the day when Hyderabad merged with the union of India, remains the black day in the history of the state. This merger was not just painful but was extremely horrifying - in every sense of the word. The "Liberation" unleashed an orgy of a human massacre which has been grossly unreported and comfortably forgotten. The tragic aftermath - statewide burnings, massive killings, looting, abduction of women and rapes - continued unabated for several days. According to the Pandit Sundarlal Committee report commissioned by the Government of India, an estimated 25 to 30 thousand people were killed, although unofficial figures range up to 200,000 deaths. The heart-rending report was never declassified and was swept under the rug for decades. It came into public domain only recently, in 2013." - A. Mohammed, Hyderabad Liberation Day: The Untold Story

I am assuming ( surely) you are talking about Hyderabad Deccan, which is now, quite unlawfully, an official state of India. Following the Indo-Pak partition in 1947, the princely states of the Indian subcontinent were asked to either accede to India or Pakistan. The British undoubtedly failed to implement a fair and honest boundary division plan, due to which most of the economic and strategically significant states were unlawfully given to India. Hyderabad Deccan was ruled by a Muslim ruler, the Nizam of Hyderabad, who had an extremely handsome fortune at the time. Hyderabad Deccan was then a prosperous state and it is reported that even the streets and pathways were regularly cleaned with perfumery, indicating its wealth. At the time of partition, the Nizam of Hyderabad decided to accede to Pakistan. But as Hyderabad is situated in the centre, surrounded on all sides by Indian territories, it seems inevitable that it will be captured. Indeed it happened. The state of Hyderabad was surrounded, a siege was laid restricting the supply of food and goods. The siege was only raised under the condition that Nizam would agree to accede to India, which he was compelled to do. The Nizam of Hyderabad thereafter retired from office as the Indians got hold of the state.