After the defeats at Poltava and Perevolotjna in 1709, more than 23 000 prisoners of war from the Swedish army were deployed across Russia, particularly in Siberia. Many were well-educated and played an important role locally. They led advanced construction work, and Swedish schools attracted many Russian pupils from leading families (often taught in German). There were conflicts, but the Swedes were appreciated. In cities like Tobolsk, the tradition lives on. Conditions are recorded in diaries and letters. Many letters from and to Swedish prisoners of war were caught by Russian censorship. Since the Russian archives where they were stored were opened in the 1990s, the letters have become accessible in principle. The project will provide Swedish, Russian and international researchers and the public with these letters. A number of letters (a total of about 900 pages) have been located in Russian archives and largely copied. After a preliminary review, a scientific analysis now follows, which mainly requires linguistic and historical expertise. An edition is the natural product. The peace of 1721 marks a turning point in Swedish history. During the following period of freedom, the country underwent an unprecedented economic, scientific and cultural boom. Sweden becomes a world leader in the natural sciences. The returning prisoners of war (a quarter) are likely to have played an important role in further developments, but their role is insufficiently studied. The hitherto censored letters offer a fortunate opportunity.