"This started out as a poem written in 1899 by Cuban writer Jose Marti. The poem is about a girl from Guantanamo and was written from the point of view of a Cuban revolutionary. In the early 1960s Pete Seeger heard Hector Angulo singing a Cuban folk song using Marti's words based on a traditional melody adapted by bandleader Joseito Fernandez. This was the time of the Cuban missile crisis and the peace activist Seeger decided to adapt it in honor of Marti. He combined Marti's original Spanish with spoken English and made it into a song for the peace movement.

In 1966, a Los Angeles folk trio called The Sandpipers recorded a version of this that hit #7 in the UK and #9 in the US. (thanks, Edward Pearce - Ashford, Kent, England, for above 2)
The song is made of 2 parts which do not have any relation whatsoever:
Part 1 - In the 19th century, an anonymous popular song circulates with the words "guarija guantanamera," which means peasant-woman from Guantanamo. It was collected and arranged in 1932 by Joseito Fernandez, who made it the hallmark of his orchestra and popularized it as a dance called "Guajira-son," which he used in place of "Bolero" in closing every ball. "La Guajira is therefore the name of a dance too.

Part 2 - It's not before 1958 that Julian Orbon combined this popular refrain with some quatrains taken at random from the immense poem Versos Sencillos (simple verses) by Jose Marti. It commences with, "Yo soy un ombre sincero," but you have to wait for hundreds of verses before finding: "Con los pobres de la tierra." At last words well in line with the acclaimed Seeger -Guthrie protest song style. Marti never mentioned any Guajira from Guantanamo in those verses. As in many popular songs, you can't find any logical link between the verses and the refrain. (thanks, Denis - Paris, France)"

from

http://www.songfacts.com/detail.php?id=4154