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 .