Buenos Aires, Argentina ? A rare guest appearance by Cuban leader Fidel Castro has turned a routine trade summit into a politically charged gathering of Washington’s greatest Latin American foes.

Castro’s surprise visit to Argentina honors the induction of Venezuela into Mercosur, the highlight of talks that start Friday. The addition gives the South American trade bloc a decidedly leftist tilt a decade after it emerged during a wave of pro-U.S. free trade sentiment.

The summit gives Latin America’s staunchest free trade critics – Castro, Venezuelan President Hugo Chavez and Bolivia’s socialist President Evo Morales – a chance to meet with the region’s more mainstream leftist leaders far beyond the influence of the White House.