This book examines and analyses the Union between the Grand Duchy of Lithuania and Sweden signed in 1655 at Këdainiai and the political crisis that followed. The author places the events in the Grand Duchy of Lithuania in the general crisis that occurred in Europe in the middle of the 17th century characterized by a great number of wars, rebellions and civil wars from Portugal to Ukraine, and which builds the background to the crisis for Lithuania and Sweden. The conflict over Lithuania and Belarus was the main reason for the Swedish-Russian, Polish-Russian and Ukrainian-Russian wars. The failure of the Union with Sweden was caused by both internal and external factors. Internally, various ethnic, confessional and political groups within the nobility of Lithuania were split in favour of different foreign powers – from Muscovy to Transylvania. The external cause for the failure of the Union project was the failure of Swedish strategy. Sweden concentrated its activity to Poland, not to Lithuania. After the Union, Swedish authorities treated the Grand Duchy as an invaded country, not an equal. The Swedish administration introduced heavy taxation and was unable to control the brutality of the army. As a result Sweden was defeated in both Lithuania and Poland. Among the different economic, political and religious explanations of the general crisis, the case of Lithuania shows the importance of the political conflicts.