The surprising return of India's Congress Party to power is potentially a major social, economic, and geopolitical development. The change of power in the world's largest democracy, which followed an election that lasted three weeks, has confounded not only outgoing Prime Minister Atal Behari Vajpayee's Bharatiya Janata Party (BJP) but also pundits, pollsters, and diplomats worldwide. Sonia Gandhi, the Italian-born widow of Rajiv Gandhi, who was assassinated in 1991, was widely expected to lose an election that was called ahead of its original date by a government that was sure of its victory. With gross domestic product growing last year...