all of which was formalized in the Treaty of Brömsebro (1645). In 1648, Sweden became a guarantor power for the Peace of Westphalia, which ended the Thirty Years' War and left her with the additional dominions of Bremen-Verden, Wismar and Swedish Pomerania. From 1638 Sweden also held the colony of New Sweden, along the Delaware River in North America. Sweden as a Great Power 1648–1721 In 1655, in the Second Northern War, Charles X Gustav of Sweden invaded and occupied western Poland–Lithuania, the eastern half of which was already occupied by Russia. The rapid Swedish advance became known in Poland as the Swedish Deluge. The Grand Duchy of Lithuania became a Swedish fief, the Polish–Lithuanian regular armies surrendered and the Polish King John II Casimir Vasa fled to the Habsburgs. The Deluge lasted for five years and took a great toll on Poland and Lithuania, with some historians crediting this invasion as the start of the downfall of Polish-Lithuanian Commonwealth. The country was devastated, treasures stolen, and insurmountable loss of lives occurred. Sweden was able to establish control of the Eastern bank of the Sound, formalised in the Treaty of Roskilde (1658), and gain recognition of her southeastern dominions by the European great powers in the Treaty of Oliva (1660); but Sweden was barred from further expansion at the Southern coast of the Baltic. Sweden came out of the Scanian War with only minor losses largely due to France forcing Sweden's adversaries into the treaties of Fontainebleau (1679) (confirmed at Lund) and Saint-Germain (1679). The following period of peace allowed Charles XI of Sweden to reform and stabilise the realm. He consolidated the finances of the Crown by the great reduction of 1680; further changes were made in finance, commerce, national maritime and land armaments, judicial procedure, church government and education. The Great Northern War: 1700 Russia, Saxony–Poland, and Denmark–Norway pooled their power in 1700 and attacked the Swedish empire. Although the young Swedish King Charles XII (1682–1718; reigned 1697–1718) won spectacular victories in the early years of the Great Northern War, most notably in the stunning success against the Russians at the Battle of Narva (1700), his plan to attack Moscow and force Russia into peace proved too ambitious. The Russians won decisively at the Battle of Poltava in June 1709, capturing much of the exhausted Swedish army. Charles XII and the remnants of his army were cut off