Brasília or Brazilia?

Brasilia
Brasilia Facts

What's the correct spelling: Brasília or Brazilia?

In modern Brazilian Portuguese, the correct way of spelling is Brasília; notice the accent over the i. In Portuguese, Brasília and Brazília are pronounced exactly the same way (as are Brasil and Brazil).

Brasília is written with an "s" because, in Portuguese, the correct spelling of the country´s name is Brasil. How come, then, that in English language, Brasil is written as Brazil?

The name brasil comes from pau-brasil, a kind of wood which once abounded in the Brazilian coast.
So, if Brasil has always been the correct form, why do English speaking countries use the variation Brazil?

This was caused by a Historical confusion.

The Brazilian Academy of Letters was founded only in 1897; until then, there was not any official institution in Brazil to issue norms on orthograhy (the Academy took over that function); besides, only a small fraction of Brazilian society had access to education.
As a consequence, as there was not an "official" or "correct" definition on how to spell it, some people used to write "Brazil", while others used "Brasil".

This lack of definition had influence even in official documents.
Let´s take, for example, the Brazilian bills of the epoch.

The bill to the left dates back to 1917.
Notice the inscription "Republica dos Estados Unidos" at the top, and the "do Brazil" right under it.
However, this other bill to the right, printed in 1921, brings the inscription "Republica dos Estados Unidos do Brasil".
This confusion lasted until 1945, when Brazil and Portugal met and agreed on the first Orthographic Vocabulary of Portuguese Language; such vocabulary defined the form "Brasil".

So, during a few decades, both Brazil and Brasil were used, even in official documents. Probably, during that period, the word "Brazil" was used more often than Brasil in English speaking countries. Even after Brazilians decided in favor of "Brasil", other countries continued using the form Brasil.