The outbreak of the war was preceded by the alliance (1699) of Peter I, Frederick IV of Denmark, and Augustus II of Poland (who was also elector of Saxony) against Charles XII, whose youth and inexperience they hoped would make him an easy victim. The war began with the invasion of Swedish Livonia by the Poles and of ducal Schleswig (which had rebelled against Danish rule with Swedish support) by the Danes. The bold and unexpected landing of Charles XII in Sjæland threatened Copenhagen and forced Denmark out of the war (1700).

Charles then turned his attention to the east; late in 1700 he routed a much superior Russian force at Narva and relieved Riga, which the Poles were besieging. Invading Poland, Charles took Warsaw and Cracow (1702), secured the election of Stanislaus I as king of Poland (1704), followed Augustus into Saxony, and forced him to break his alliance with Russia and to recognize Stanislaus as king by the Treaty of Altranstädt (1706). While Charles was victorious in Poland, however, Peter I occupied Ingermanland and part of Livonia.

Resuming (1707) his campaign against Russia, Charles invaded Ukraine, where Mazepa had promised to foment an anti-Russian uprising. Mazepa's project failed, and the Swedes, cut off from reinforcements and in need of a stronghold, laid siege to the fortress of Poltava. There a superior Russian army utterly defeated (1709) the Swedes, and Charles retired with a handful of men to Bessarabia, on Turkish territory.

His intrigues at Constantinople induced the sultan to declare war on Russia (1710). Peter I, allied with Prince Constantine Brancovan of Walachia and Prince Demetrius Cantemir of Moldavia, invaded these two vassal principalities of Turkey and entered Jassy, but he soon found himself outnumbered and consented (1711) to the disadvantageous Treaty of the Pruth (see Russo-Turkish Wars).

While Charles was stubbornly refusing to leave Turkey, Augustus II took advantage of his absence; he invaded (1709) Poland and expelled Stanislaus I, while Peter I completed the conquest of Swedish Livonia, Ingermanland, and Karelia. Frederick IV of Denmark also resumed the war, seized ducal Schleswig, and conquered the Swedish duchies of Bremen and Verden in Germany, which he sold to Hanover on condition that Hanover join in the war against Sweden. Swedish Pomerania was taken by the Poles, and Prussia, fishing in troubled waters, seized Stettin. In 1714, Charles XII returned to Sweden. Undaunted by the coalition of Russia, Denmark, Poland, Saxony, Hanover, and Prussia, he began military operations in Norway (then ruled by Denmark), where he was fatally shot in 1718.

Charles's successor, Ulrica Leonora, and her husband, Frederick I of Sweden, began peace negotiations. Peace was made with all enemies but Russia in the treaties of Stockholm and Frederiksborg (1719–20). Augustus II of Poland restored all his conquests; Hanover retained the duchies of Bremen and Verden, but paid a large indemnity; Prussia received Stettin and part of W Pomerania, the rest reverting to Sweden; Denmark restored its conquests for a payment, but Sweden permitted the union of ducal Schleswig with royal Schleswig under the Danish crown and renounced Swedish exemption from customs duties in the Sound. By the Treaty of Nystad with Russia (1721) Sweden ceded Livonia (including Estonia), part of Karelia, and Ingermanland, but retained Finland. The lasting results of the Northern War were the waning of Swedish power, the establishment of Russia as a major power of Europe, with its "window" on the Baltic Sea, and the decay of Poland.

See L. Cooper, Many Roads to Moscow (1968).