The 1941 census had estimated the population of Hyderabad to be 16.34 million, over 85% of who were Hindus and Muslims about 12%.
In spite of Hindu majority, Hindus were severely under-represented, The Nizam and his nobles, mostly Muslims owned 40% of the total land in the state

After independence
In 1947 after independence the British raj left the local rulers of the princely states the choice of whether to join or or to remain independent.
Nizam Osman Ali Khan decided not to join either or after the partition of India.

The Nizam's defiance was backed by armed militias, known as Razakars. Md Ali Jinnah came to Hyderabad in 1947 and made many provocative statements addressing public meeting.

He said, “We will break the Hindus like chicken necks. We will cut them like radish” which put the Razakars on an aggressive mode.

Who are Razakars -
In 1926, Mahmud Nawaz Khan, a retired Hyderabad official, founded the Majlis-e-Ittehad-ul-Muslimeen (also known as MIM).

"Its objectives were to unite the Muslims in the State in support of Nizam and to reduce the Hindu majority by large-scale conversion to Islam".
Later it was transformed into religio-political outfit by Nawab Bahadur Yar Jung & Qasim rizvi,

after death of Jung, Rizvi became president of MIM & raised a militia named "Razakars". Fearing a Hindu civil uprising in his own kingdom, the Nizam allowed to set up a voluntary militia of Muslims.

The conflict -
Qasim Razvi, a Muslim educated at AMU who claimed Hyderabad was a Muslim state and that Muslim supremacy was based upon the right of conquest" and had the moral support of Pakistan.

In the Hindu majority Hyderabad, the Nizam had ensured that Hindus didn’t enjoy even the basic fundamental rights. The Razakars who numbered up to 200,000 at the height of the conflict swore to uphold Islamic domination in Hyderabad,

against  the majority Hindu population favouring the accession of Hyderabad into the India.
there was an escalation of violence between the Razakars and Hindu communities. In more than 150 villages (of which 70 were in Indian territory outside Hyderabad) were pushed into violence

From the beginning of 1948 the Razakars had extended their activities from Hyderabad city into the towns and rural areas, murdering hindus, abducting women, pillaging houses and fields, and looting non-muslim property in a widespread reign of terror.

It is reported that the Nizam received arms supplies from Pakistan and from the Portuguese administration based in Goa. In addition, additional arms supplies were received via airdrops from an Australian arms trader Sidney Cotton.

The Nizam then entered into a double-game, while at one hand he was building his army, he also started peace negotiations with the government of India. He also started attacking Hindus by spreading hatred against them in the name of religious `Jihad’.

The razakars attacked many villages in Telangana region, looting people, raping women and conducting massacres, and spread the reign of terror.

in the `peace committee’ meeting once, Sri Satthagopacharyulu condemned the extreme atrocities of razakars, the very next day he was dragged to the Janagama road and shot dead.

terrifyingly incident, as told by MN Reddy; 6 kms away from Inanoor police station, and he saw five corpses hung on a tamarind tree, the bodies are of five Brahmins. A group of seven Brahmins were returning after attending a death ceremony they were killed Razakar gang.

Razakars and govt staff together used to raid villages and loot them during nights without any qualms. They used to pull and snatch ear-rings and nose rings of women, and used to leave them bleeding.

So in this Barberic killing spree like (ancestors) Mughal invaders Razakars and Nizam govt. butchered more than 40000 non muzlims in a span of 1-1.5 year, for creating an independent islamic state within Indian Border.

Operation Polo -
Sardar Patel described the idea of an independent Hyderabad as an ulcer in the heart of India - which had to removed surgically. 
Sardar Patel responded later by stating "If you threaten us with violence, swords will be met with swords"

At 4 a.m. on 13 September 1948, India's Hyderabad Campaign, code-named "Operation Polo" by the Indian Army led by Major General J. N. Chaudhuri, began. Indian troops invaded Hyderabad from all points of the compass.

Secretary-General of the Hyderabad Department of External Affairs in a cablegram informed the United Nations Security Council that Hyderabad was being invaded by Indian forces and that hostilities had broken out.

At 5 p.m. on 17 September 1948 the Nizam's army surrendered. India then incorporated the state of Hyderabad and ended the rule of the Nizams.
Eventually, the Indian Army routed the Razakars during Operation Polo.

Qasim Razvi was initially jailed and then allowed to move to Pakistan where he was granted asylum, on an undertaking that he would migrate to Pakistan within forty-eight hours of his release.
On 22 September 1948, the Nizam withdrew his complaint from the UN Security Council.

AIMIM -
Majlis-e-Ittehadul Muslimeen (MIM) was banned, it was rechartered under the Congress govt. as All India MIM (AIMIM) by Abdul Wahed Owaisi in 1957. 
He was arrested in March 1958 for making hate speeches against in public meetings. He was prisoned for 11 months in Jail.

S.S. Owaisi, father Akbaruddin Owaisi challenged the detention of his father but court dismissed the petition. After death of Abdul Wahed he became the president of AIMIM in 1976.

After death of Abdul Wahed he became the president of AIMIM in 1976.
Before retiring S.S.Owaisi placed his both his sons as MP and MLA.

Source for Pics of Operation Polo
https://www.merepix.com/2013/03/operation-polo-hyderabad-police-action-1948-photos.html?m=1