Córdoba, Argentina -- By late last Friday afternoon in Argentina's second largest city, the bi-annual summit of Mercosur had officially wrapped. But on a dusty rugby field at a nearby university, the changing face of this South American economic alliance was on full display. "North American imperialism is endangering the welfare of the human species on this earth," said Venezuelan President Hugo Chávez, addressing thousands of chanting, banner-waving demonstrators who had come to hear him speak along with Cuban President Fidel Castro. "Imperialism is international, and the solution must be international, too." His remarks came on the closing day of Venezuela's inaugural appearance as the fifth member of Mercosur, a regional agreement aimed at fostering free trade and economic integration in South America. The trade bloc was first established by Argentina, Brazil, Paraguay, and Uruguay in 1991. For Chávez, membership in Mercosur means more than just favorable trade conditions. His...