If anyone cared to play Wc3 and finish the campaign they would know...but here it is briefly
Thrall the Warchief entered Kalimdor to try and help his friend Grom Hellscream from the Demonic possession that him and the orcs had once been put upon them. At the same time, the Humans were here to explore the new land that had been discovered...led by Jaina Proudmoore (A former friend of Arthas - The leader of the Scourge)
During this time, the Nelfs, Humans and orcs formed an alliance at the base of Mount Hyjal to stop the Scourge and the Demonic Legion from destroying the "earth" by protecting the Great tree. Archimonde the Demon was defeated a long with numerous other Demonic leaders.
However, soon the truce broke, after Jainas Father (Admiral Proudmoore) sent troops to Kalimdor to try and execute Thrall.
Illidan - Cenarius's brother, tried to take the throne of the Lich king, and with his Naga allies, fought with the scourge, as the nelf race was pwoerless to do anything. Whoever they helped, would come out stronger than anyone and claim the throne as dominant race, so the Night elfs withdrew from the battle and set an alliance with the Humans - who, always had Dwarfs in their ranks (The Mountain King Hero, Gryphon Riders etc.)
Not quite sure where gnomes came in, because they played no part in Wc3 Reign of Chaos or The Frozen Throne...
Sylvannus wished to lead a seperate life, after bein killed by the witch Kings Blade, which turned her into a Banshee...her and her fellow forsaken joined the Horde, through convenience