Original: This operation, termed Operation Polo, was successful and on 17 September 1948, the Nizam signed an Instrument of Accession to the Union of India. The Constitution of India, which went into effect on 26 January 1950, made Hyderabad state one of the part B states of India and Hyderabad city continued to be its capital.
Simplified: But soon the Nizam had to sign an Instrument of Accession to the Union of India so that Hyderabad becomes a part of India.