Charles XII of Sweden stops his march to conquer Moscow outside Smolensk, marking the turning point in the Great Northern War. The army is defeated nine months later in the Battle of Poltava, and the Swedish Empire ceases to be a major power.

The Battle of Poltava (8 July 1709) was the decisive and largest battle of the Great Northern War and a catastrophe for Sweden. A Russian army under the command of Tsar Peter I defeated a Swedish army, under the command of Carl Gustaf Rehnskild. The battle put an end to the status of the Swedish Empire as a European great power, as well as its eastbound expansion, and marked the beginning of Russian hegemony in Northern Europe.

During the course of six years in the initial stages of the war, King Charles XII and the Swedish Empire had defeated almost all participants in the anti-Swedish coalition, which initially consisted of the Polish-Lithuanian Commonwealth, Denmark-Norway and the Tsardom of Russia. The latter under Tsar Peter I's rule was the only one still undefeated. Charles XII therefore chose to invade Russia in the autumn of 1707 and march towards Moscow with a large Swedish army. However, the campaign was complicated by harsh weather conditions and by the Russians applying scorched earth tactics:704 and surprise attacks, which forced Charles XII to interrupt his march on Moscow and instead march south to establish winter quarters in Ruthenia with the help of Ivan Mazepa, hetman of the Cossack Hetmanate Zaporizhian Host.

After the extremely harsh Great Frost of 17081709, the weakened Swedish army resumed operations in the spring of 1709 and began a siege of the fortress of Poltava, an important trading center and military depot on the Vorskla. Meanwhile, a numerically superior Russian army of 75,00080,000 men commanded by Peter, advanced to Poltava in order to relieve the siege. The two armies clashed, and ended with the Swedes being defeated and fleeing the battlefield. Charles and Mazepa retreated with 1,500 men south to the river Dnieper, which they crossed, thus managing to escape the Russians and established themselves in the Ottoman Empire.:710 The rest of the army was forced to surrender to the Russians at the village of Perevolochna on 11 July 1709.The Battle of Poltava, as well as the subsequent capitulation, ended in a decisive victory for Peter I and became the greatest military catastrophe in Swedish history. It marked a turning point in the continuation of the war in favour of the anti-Swedish coalition, which as a result of the battle was revived and with renewed vigor attacked the weakened Swedish Empire on several fronts. The battle thus marked the end of Sweden's time as the dominant power in Northern Europe, a position which after the war was taken over by the Russian Empire. The battle is therefore of crucial importance in the history of Sweden as well as Russia and Ukraine.

Charles XII, sometimes Carl XII (Swedish: Karl XII) or Carolus Rex (17 June 1682 – 30 November 1718 O.S.), was King of Sweden (including current Finland) from 1697 to 1718. He belonged to the House of Palatinate-Zweibrücken, a branch line of the House of Wittelsbach. Charles was the only surviving son of Charles XI and Ulrika Eleonora the Elder. He assumed power, after a seven-month caretaker government, at the age of fifteen.In 1700, a triple alliance of Denmark–Norway, Saxony–Poland–Lithuania and Russia launched a threefold attack on the Swedish protectorate of Holstein-Gottorp and provinces of Livonia and Ingria, aiming to draw advantage as the Swedish Empire was unaligned and ruled by a young and inexperienced king, thus initiating the Great Northern War. Leading the Swedish army against the alliance, Charles won multiple victories despite being usually significantly outnumbered. A major victory over a Russian army some three times the size in 1700, at the Battle of Narva, compelled Peter the Great to sue for peace, an offer that Charles subsequently rejected. By 1706, Charles, now 24 years old, had forced all of his foes into submission including, in that year, a decisively devastating victory by Swedish forces under general Carl Gustav Rehnskiöld over a combined army of Saxony and Russia at the Battle of Fraustadt. Russia was now the sole remaining hostile power.

Charles's subsequent march on Moscow met with initial success as victory followed victory, the most significant of which was the Battle of Holowczyn where the smaller Swedish army routed a Russian army twice the size. The campaign ended with disaster when the Swedish army suffered heavy losses to a Russian force more than twice its size at Poltava. Charles had been incapacitated by a wound prior to the battle, rendering him unable to take command. The defeat was followed by the Surrender at Perevolochna. Charles spent the following years in exile in the Ottoman Empire before returning to lead an assault on Norway, trying to evict the Danish king from the war once more in order to aim all his forces at the Russians. Two campaigns met with frustration and ultimate failure, concluding with his death at the Siege of Fredriksten in 1718. At the time, most of the Swedish Empire was under foreign military occupation, though Sweden itself was still free. This situation was later formalized, albeit moderated in the subsequent Treaty of Nystad. The result was the end of the Swedish Empire, and also of its effectively organized absolute monarchy and war machine, commencing a parliamentary government unique for continental Europe, which would last for half a century until royal autocracy was restored by Gustav III.Charles was an exceptionally skilled military leader and tactician as well as an able politician, credited with introducing important tax and legal reforms. As for his famous reluctance towards peace efforts, he is quoted by Voltaire as saying upon the outbreak of the war; "I have resolved never to start an unjust war but never to end a legitimate one except by defeating my enemies". With the war consuming more than half his life and nearly all his reign, he never married and fathered no children. He was succeeded by his sister Ulrika Eleonora, who in turn was coerced to hand over all substantial powers to the Riksdag of the Estates and opted to surrender the throne to her husband, who became King Frederick I of Sweden.