The Great Northern War was a conflict between Sweden and a Russian-led coalition between 1700 and 1721. In it, the Russian forces won after a long fight, and Sweden had to give Karelia, Ingria, and Livonia to the Russians. This was mostly because the Swedish allies left early in the war, and Russia had more allies than Sweden.
but what if:
- Saxony had been part of the Swedish alliance, and Poland had been since the beginning,
- and Poland was in a more stable state that it was on OTL?
Eventually, the Great Northern War ends in the disintegration of Denmark-Norway, and a great weakening of Russia with the loss of Karelia, Kola, and northwestern Russia. This brings to a further strenghtening of the Swedish Empire, and an eventual transformation of it into the world's superpower.
Poland joins the Great Northern War in 1700, on Sweden's side. Saxony, bound by a personal union with Poland, joins the War the same year. This allows the Swedish army to maintain centered in the Baltic instead of having to break between Poland and Russia after the victory in Narva in 1703. This leads to a crushing victory over Peter I's armies in the Battle of Tver in early 1704.
The war draws out until 1725, when it finally ends after the Swedish capture of Moscow and the Treaties of Dresden.