Peter the Great Defeats Charles XII at Poltava

In the summer of 1709, King Charles XII of Sweden sought a decisive engagement against Tsar Peter I deep inside Ukrainian territory. Charles had invaded the Russian realm in 1708 after years of campaigning across Poland and Saxony. In 1706 and 1707, Peter had offered through intermediaries to return much of his Baltic conquests in exchange for keeping the Neva region of Ingria, where the tsar had already begun constructing his intended capital of St. Petersburg. Charles rejected the proposal, aiming instead to march eastward toward Moscow. In response, Russian commanders implemented scorched-earth tactics that deprived the Swedish invaders of food, forage, and secure lines of communication, eventually turning the campaign away from the Russian interior and toward Ukraine.

The Swedish invasion suffered catastrophic attrition during the winter of 1708 to 1709, which proved to be the coldest recorded across Europe in centuries. Charles had allied himself with Ivan Mazepa, hetman of the Cossack Hetmanate in Ukraine, but Russian forces struck preemptively by sacking Mazepa's headquarters at Baturyn and seizing vital provisions. By the spring of 1709, the Swedish field army had shrunk to somewhere between about twenty thousand and thirty thousand men, depending on whether irregulars are counted, and it had few guns. Hoping to draw Peter into an open confrontation and secure a depot on the Vorskla River, Charles laid siege to the fortress of Poltava in May 1709. Peter soon arrived to relieve the garrison, establishing a fortified encampment north of the town with an army that outnumbered the Swedes (the Encyclopedia of Ukraine counts about 42,500 Russian soldiers in the battle).