RIO DE JANEIRO — A massive power failure blacked out Brazil's two largest cities and other parts of Latin America's biggest nation for more than two hours late Tuesday, leaving millions of people in the dark after a huge hydroelectric dam suddenly went offline. Paraguay was also affected when the Itaipu dam straddling the two nations' border stopped producing 17,000 megawatts of power, resulting in outages in Rio de Janeiro, Sao Paulo and at least several other big Brazilian cities, Brazilian Mines and Energy Minister Edison Lobao said. The cause of the failure had not been determined, but Lobao said...