Complex sentence: The Swedish invasion of Russia began in 1707 and ended with the disastrous Swedish defeat at Poltava which resulted in the decisive turn around of the war with Saxony and Denmark rejoining the alliance.
Split into simpler sentences: The Swedish invasion of Russia began in 1707 and ended with the disastrous Swedish defeat at Poltava. This marked a decisive turn in the war with Saxony and Denmark rejoining the alliance.