With mostly stagnant economies and stalled trade, South American leaders have had little to celebrate. So when they get together, as they did this week for the Mercosur trade summit in Argentina, what fills the gap are feel-good speeches about Latin camaraderie leavened with tub-thumping against gringo imperialism.

That script is about to change.

Argentine President Cristina Kirchner interrupted the speaker's list to break the news of the agreement to re-establish diplomatic relations between Cuba and U.S., just then being unveiled in simultaneous announcements by Cuba's President Raul Castro, in Havana, and U.S. President Barack Obama, in Washington. "For those of us with gray hair," she said, "this is a historical moment."