After the Swedish defeats in 1709 in what is now Ukraine, 23 000 Carolinians were taken into Russian captivity. They ended up in various parts of Russia, including Siberia. Many were well educated and were needed for various tasks. They organized building projects and founded schools that also attracted Russian students from leading families. The Swedes were generally appreciated. Many went into hiding after the end of the peace, while others remained voluntarily. The vast majority, however, were constantly longing for home. Some fled, many died of starvation and hardship.

The Carolinians' life in captivity is described in diaries and letters, some of which have been published. However, several letters never reached their addressees in Sweden or Russia because they were caught up in the censorship. But many were archived, and these can now be studied after the Russian archives were opened in the 1990s.

With financial support from the Torsten Söderberg Foundation, the project was carried out in 2015, 2017, and 2018. Previously unknown letters were located in Russian archives, copied, and digitized. Scholarly analysis is currently underway. More letters than expected have been discovered. These are of a constitutional and national political nature or purely private. They are written in Swedish and German, some in French or Latin. Since their contents were previously unknown, they are expected to shed new light on the conditions of the time. After the peace of 1721, a new Sweden emerged. The letters were written during a decisive period of transition.