In the Great Northern War (1700–1721) a coalition led by Russia successfully contested the supremacy of Sweden in Northern, Central and Eastern Europe. The initial leaders of the anti-Swedish alliance were Peter I of Russia, Frederick IV of Denmark–Norway and Augustus II the Strong of Saxony-Poland-Lithuania. Frederick IV and Augustus II were defeated by Sweden, under Charles XII, and forced out of the alliance in 1700 and 1706, respectively, but rejoined it in 1709 after the defeat of Charles XII at the Battle of Poltava. George I of Great Britain and the Electorate of Hanover joined the coalition in 1714 for Hanover and in 1717 for Britain, and Frederick William I of Brandenburg-Prussia joined it in 1715.
Charles XII led the Swedish Army. Swedish allies included Holstein-Gottorp, several Polish magnates under Stanisław I Leszczyński (1704–1710) and Cossacks under the Ukrainian Hetman Ivan Mazepa (1708–1710). The Ottoman Empire temporarily hosted Charles XII of Sweden and intervened against Peter I.
The war began when Denmark–Norway, Saxony and Russia, sensing an opportunity as Sweden was ruled by the young Charles XII, formed a coalition against Sweden. Denmark invaded Sweden's ally Holstein-Gottorp, while Saxony and Russia declared war on the Swedish Empire and attacked Swedish Livonia and Swedish Ingria, respectively. Sweden parried the Danes at Travendal (August 1700) and the Russians at Narva (November 1700), and in a counter-offensive pushed Augustus II's forces through the Polish–Lithuanian Commonwealth to Saxony, dethroning Augustus on the way (September 1706) and forcing him to acknowledge defeat in the Treaty of Altranstädt (October 1706). The treaty also secured the extradition and execution of Johann Reinhold Patkul, architect of the alliance seven years earlier. Meanwhile, the forces of Peter I had recovered from defeat at Narva and gained ground in Sweden's Baltic provinces, where they cemented Russian access to the Baltic Sea by founding Saint Petersburg in 1703. Charles XII moved from Saxony into Russia to confront Peter, but the campaign ended in 1709 with the destruction of the main Swedish army at the decisive Battle of Poltava (in present-day Ukraine) and Charles' exile in the Ottoman town of Bender. The Ottoman Empire defeated the Russian–Moldavian army in the Pruth River Campaign, but that peace treaty was in the end without great consequence to Russia's position.
After Poltava, the anti-Swedish coalition revived and subsequently Hanover and Prussia joined it. The remaining Swedish forces in plague-stricken areas south and east of the Baltic Sea were evicted, with the last city, Tallinn, falling in the autumn of 1710. By 1714, Russia had occupied Finland.
On 18 October 1709, Sweden received Denmark-Norway’s declaration of war, presented as a response to the alleged maltreatment of Danes by Sweden in Scania. Swedish victory at Helsingborg amounted to only a brief respite before ultimate defeat in the west.
From June 1716, Denmark and Russia prepared a joint invasion of Sweden proper, which the Tsar envisioned as dealing the Swedish king a decisive blow. Earlier that year, Charles XII had invaded south-eastern Norway, but a string of successive defeats, together with the systematic disruption of Swedish logistics by the Norwegians, had forced him back towards the border. In June and July, a renewed offensive against Fredrikshald, or Halden, was frustrated by the combined efforts of the town's garrison and inhabitants. The campaign was finally abandoned after Tordenskjold, the Norwegian naval hero, captured almost the entire Swedish supply and transport fleet at Dynekilen, forcing Charles XII to finally retreat across the border. In the meantime, a Danish-Russian invasion force was assembling across the strait. Severe delays, however, pushed the operation into autumn and, together with growing distrust and strategic divergence between the allied monarchs, ultimately led to its cancellation.
In 1718, Sweden launched a second and even larger campaign against Norway. Charles XII was ultimately killed in the trenches before Fredriksten, while a second Swedish field army was forced onto the Carolean Death March during its retreat from Trøndelag, losing most of its men. The failure of the Norwegian campaign constituted a severe strategic defeat for Sweden and effectively brought its offensive war to an end.
The war ended with the defeat of Sweden, leaving Russia as the new dominant power in the Baltic region and as a new major force in European politics. The Western powers, Great Britain and France, became caught up in the separate War of the Spanish Succession (1701–1714), which broke out over the Bourbon Philip of Anjou's succession to the Spanish throne and a possible joining of France and Spain. The formal conclusion of the Great Northern War came with the Swedish-Hanoverian and Swedish-Prussian Treaties of Stockholm (1719), the Dano-Swedish Treaty of Frederiksborg (1720), and the Russo-Swedish Treaty of Nystad (1721). By these treaties Sweden ceded its exemption from the Sound Dues and lost the Baltic provinces and the southern part of Swedish Pomerania. The peace treaties also ended its alliance with Holstein-Gottorp. Hanover gained Bremen-Verden, Brandenburg-Prussia incorporated the Oder estuary (Stettin Lagoons), Russia secured the Baltic Provinces, and Denmark strengthened its position in Schleswig-Holstein. In Sweden, the absolute monarchy had come to an end with the death of Charles XII, and Sweden's Age of Liberty began.
Between 1560 and 1658, Sweden created a Baltic empire centred on the Gulf of Finland and comprising the provinces of Karelia, Ingria, Estonia, and Livonia. During the Thirty Years' War, Sweden gained tracts in Germany as well, including Western Pomerania, Wismar, and the duchies of Bremen and Verden. During the same period, Sweden conquered Danish and Norwegian provinces north of the Sound (1645; 1658). These victories may be ascribed to a well-trained army, which, despite its comparatively small size, was far more professional than most continental armies, and also to a modernization of administration (both civilian and military) in the course of the 17th century, which enabled the monarchy to harness the resources of the country and its empire effectively. Fighting in the field, the Swedish Army (which during the Thirty Years' War contained more German and Scottish mercenaries than ethnic Swedes, but was administered by the Swedish Crown) was able, in particular, to make quick, sustained marches across large tracts of land and to maintain a high rate of small arms fire due to proficient military drill.
However, the Swedish state ultimately proved unable to support and maintain its army in a prolonged war. Campaigns on the continent had been proposed on the basis that the army would be financially self-supporting through plunder and taxation of newly gained land, a concept shared by most major powers of the period. The cost of the warfare proved to be much higher than the occupied countries could fund, and Sweden's coffers and resources in manpower were eventually drained in the course of long conflicts.
The foreign interventions in Russia during the Time of Troubles resulted in Swedish gains in the Treaty of Stolbovo (1617). The treaty deprived Russia of direct access to the Baltic Sea. Russian fortunes began to reverse in the final years of the 17th century, notably with the rise to power of Peter the Great, who looked to address the earlier losses and re-establish a Baltic presence. In the late 1690s, the adventurer Johann Patkul managed to ally Russia with Denmark and Saxony by the secret Treaty of Preobrazhenskoye, and in 1700 the three powers attacked.
Charles XII of Sweden succeeded Charles XI of Sweden in 1697, aged 14. From his predecessor, he took over the Swedish Empire as an absolute monarch. Charles XI had tried to keep the empire out of wars, and concentrated on inner reforms such as reduction and allotment, which had strengthened the monarch's status and the empire's military abilities. Charles XII refrained from all kinds of luxury and alcohol and usage of the French language, since he considered these things decadent and superfluous. He preferred the life of an ordinary soldier on horseback, not that of contemporary baroque courts. He determinedly pursued his goal of dethroning his adversaries, whom he considered unworthy of their thrones due to broken promises, thereby refusing to take several chances to make peace. During the war, the most important Swedish commanders besides Charles XII were his close friend Carl Gustav Rehnskiöld, also Magnus Stenbock and Adam Ludwig Lewenhaupt.