Why did Charles 12 manage to quickly defeat the forces of Denmark, Saxony and Russia at the beginning of the war?

This is due to the fact that the Swedish army was very strong and well-armed. Thus, the troops of the Danes, Saxons and Russians in the early years of the war suffered one defeat after another. The situation in the war was changed only after the Battle of Poltava, then Peter 1 was finally able to take the initiative into his own hands.