In 1645, a bloody war raged between Dutch settlers and the Portuguese empire over the sugar plantations of north-east Brazil.

Trapped on either side of the conflict were the Potiguara, a powerful indigenous nation whose leaders penned a series of letters in the Tupi language, enticing their relatives to defect across enemy lines.

Now, a painstaking new translation of the correspondence has been hailed as a “huge achievement” in casting new light on these unique sources written by a native people.

The forthcoming publication is the fruit of 30 years of work by Eduardo de Almeida Navarro, a specialist in classical indigenous languages at the University of São Paulo.

“It’s hugely exciting to be able to make this contribution to the history of my country,” said Navarro.

The letters were first uncovered in the Dutch archives in 1885, but the texts were blotted and jumbled. Many words were not in existing glossaries of Tupi, which gives us words like piranha and jaguar. In 1906, one frustrated translator called the letters “genuine enigmas”.

Navarro spent decades compiling a comprehensive ancient Tupi dictionary, drawing on the accounts of French traders and English buccaneers. This helped him fully translate the letters, revealing the desperate efforts of the Potiguara chiefs to save their people from destruction.

“Why,” wrote Felipe Camarão, a Potiguara captain fighting for Portugal, “do I make war against people of our own blood? … Come to me and I will forgive you. I will make you one with your ancient culture again. Those that stay there will be destroyed.”