While Charles won several decisive battles in the Commonwealth and ultimately secured the coronation of his ally Stanisław Leszczyński and the surrender of Saxony, the Russian Tsar Peter the Great embarked on a military reform plan that improved the Russian army, using the effectively organized Swedes and other European standards for role models.
Russian forces managed to penetrate Ingria and established a new city, Saint Petersburg, there.
Charles planned an invasion of the Russian heartland, allying himself with Ivan Mazepa, Hetman of the Ukrainian Cossacks.
The size of the invading Swedish army was peeled off as Charles left Leszczyński with some 24,000 German and Polish troops, departing eastwards from Saxony in late 1707 with some 35,000 men, adding a further 12,500 under Adam Ludwig Lewenhaupt marching from Livonia.
Charles left the homeland with a defense of approximately 28,800 men, with a further 14,000 in Swedish Finland as well as further garrisons in the Baltic and German provinces.

After securing his "favorite" victory in the Battle of Holowczyn, despite being outnumbered over three to one against the new Russian army, Charles opted to march eastwards on Moscow rather than try to seize Saint Petersburg, founded from the Swedish town of Nyenskans five years earlier.
Peter the Great managed, however, to ambush Lewenhaupt's army at Lesnaya before Charles could combine his forces, thus losing valuable supplies, artillery and half of Lewenhaupt's men.
Charles' Polish ally, Stanisław Leszczyński, was facing internal problems of his own.
Charles expected the support of a massive Cossack rebellion led by Mazepa in Ukraine, with estimates suggesting Mazepa of being able to muster some 40,000 troops, but the Russians subjugated the rebellion and destroyed its capital Baturin before the arrival of the Swedish troops.
The harsh climate took its toll as well, as Charles marched his troops for winter camp in Ukraine.

By the time of the decisive Battle of Poltava, Charles had been wounded, one-third of his infantry was dead, and his supply train was destroyed.
The king was incapacitated by a coma resulting from his injuries and was unable to lead the Swedish forces.
With the numbers of Charles' army reduced to some 23,000, with several wounded and handling the siege of Poltava, his general Carl Gustav Rehnskiöld had a clearly inferior force to face the fortified and modernized army of Tsar Peter, with some 45,000 men.
The Swedish assault ended in disaster, and the king fled with a small entourage south to the Ottoman Empire, where he set up camp at Bender with some 1,000 of his Caroleans ("Karoliner" in Swedish).
The remainder of the army surrendered days later at Perevolochna under Lewenhaupt's command, most of them (including Lewenhaupt himself) spending the rest of their days in Russian captivity.

The Swedish defeat at Poltava in 1709 marks the downfall of the Swedish Empire, as well as the founding of the Russian Empire.
While Charles won several decisive battles in the Commonwealth and ultimately secured the coronation of his ally Stanisław Leszczyński and the surrender of Saxony, the Russian Tsar Peter the Great embarked on a military reform plan that improved the Russian army, using the effectively organized Swedes and other European standards for role models. Russian forces managed to penetrate Ingria and established a new city, Saint Petersburg, there. Charles planned an invasion of the Russian heartland, allying himself with Ivan Mazepa, Hetman of the Ukrainian Cossacks. The size of the invading Swedish army was peeled off as Charles left Leszczyński with some 24,000 German and Polish troops, departing eastwards from Saxony in late 1707 with some 35,000 men, adding a further 12,500 under Adam Ludwig Lewenhaupt marching from Livonia. Charles left the homeland with a defense of approximately 28,800 men, with a further 14,000 in Swedish Finland as well as further garrisons in the Baltic and German provinces.
After securing his "favorite" victory in the Battle of Holowczyn, despite being outnumbered over three to one against the new Russian army, Charles opted to march eastwards on Moscow rather than try to seize Saint Petersburg, founded from the Swedish town of Nyenskans five years earlier. Peter the Great managed, however, to ambush Lewenhaupt's army at Lesnaya before Charles could combine his forces, thus losing valuable supplies, artillery and half of Lewenhaupt's men. Charles' Polish ally, Stanisław Leszczyński, was facing internal problems of his own. Charles expected the support of a massive Cossack rebellion led by Mazepa in Ukraine, with estimates suggesting Mazepa of being able to muster some 40,000 troops, but the Russians subjugated the rebellion and destroyed its capital Baturin before the arrival of the Swedish troops. The harsh climate took its toll as well, as Charles marched his troops for winter camp in Ukraine.
By the time of the decisive Battle of Poltava, Charles had been wounded, one-third of his infantry was dead, and his supply train was destroyed. The king was incapacitated by a coma resulting from his injuries and was unable to lead the Swedish forces. With the numbers of Charles' army reduced to some 23,000, with several wounded and handling the siege of Poltava, his general Carl Gustav Rehnskiöld had a clearly inferior force to face the fortified and modernized army of Tsar Peter, with some 45,000 men. The Swedish assault ended in disaster, and the king fled with a small entourage south to the Ottoman Empire, where he set up camp at Bender with some 1,000 of his Caroleans ("Karoliner" in Swedish). The remainder of the army surrendered days later at Perevolochna under Lewenhaupt's command, most of them (including Lewenhaupt himself) spending the rest of their days in Russian captivity.
The Swedish defeat at Poltava in 1709 marks the downfall of the Swedish Empire, as well as the founding of the Russian Empire.
Charles XII of Sweden