This book examines and analyses the Union between the Grand Duchy of Lithuania and Sweden signed in 1655 at K?dainiai and the political crisis that followed. The union was a result of strong separatist dreams among the Lithuanian-Ruthenian Protestant elite led by the Radziwi?? family, and if implemented it would radically change the balance of power in the Baltic Sea region. The main legal point of the Union was the breach of Lithuanian federation with Poland and the establishment of a federation with Sweden. The Grand Duchy of Lithuania aspired to return to international relations as a self-governing subject. The Union meant a new Scandinavian alternative to Polish and Russian domination. 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 research proved the importance of lesser powers in changing the geopolitical balance between the Great Powers.