This book examines and analyses the Union between the Grand Duchy of Lithuania and Sweden signed in 1655 at Kėdainiai and the political crisis that followed. The union was a result of strong separatist dreams among the Lithuanian-Ruthenian Protestant elite led by the Radziwiłł 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. 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. For the separatists of Lithuania the main motive to turn against Poland and to promote alliance with Sweden, Russia or the Cossacks was the inability of Poland to shield the Grand Duchy from a Russian invasion.The Lithuanian case was a provincial rebellion led by the native nobility against their monarch, based on tradition of the previous independence and statehood period. It was not nationalism in its modern meaning, but instead a crisis of identity in the form of a conflict between Patria and Central Power. However, the cost of being a part of Sweden or Muscovy was greater than the benefit of political protection. Therefore, the pro-Polish orientation prevailed when Poland after 1658 recovered its military ability the local nobility regrouped around Warsaw. The Grand Duchy of Lithuania managed to remain on the political map of Europe, but at the price of general religious Catholization and cultural Polonization. After the crisis, the Grand Duchy of Lithuania gradually changed into a deep province of the Polish state.