The problem of the [Princely] states is so difficult
that only you can solve it.
—Gandhi to Sardar Patel.

Sir Stafford Cripps had estimated it would take India 10 to 15 years or more, to merge the States with India. It was a surprise to all, and a tribute to the Sardar’s abilities, that he took, not 10 to 15 years, but less than 10 to 15 months to merge all the 548 Princely States with India, extending India’s geographical area by a whopping 40%.
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This book comprehensively covers the Integration of the Indian Princely States into India.

India of today is what was British India after partition, plus additional 40% geographical area on account of the integration of 548 Princely States into it (out of 562, 14 went to Pakistan), which Britain had declared to be independent upon lapse of their paramountcy. The credit for expanding India’s area...