Lore-wise speaking here it is...
The Horde [Which at the time was only the Orcs on Draenor, their home planet.] They were corrupted by Kil'jaeden. After pretty much obliterating all other life on their planet so only they Truly ruled, the Orc's corruption and blood lust eventually caused several groups to split, and they waged war on each other. One thing led too another and they all allied and the another thing [I'm skipping the unnecessary stuff] and they were able to open a rift onto Azeroth. They invaded Azeroth, and at the time they were south of the Humans. [Blasted Lands / The Dark Portal]. They attacked Stormwind, despite being told NOT TO at the time. The Humans obviously countered with Lordaeron. By this time, when the Second War came, Doomhammer basically was in control. At the time, once about to attack Lordaeron, 1/3 of the Horde broke for a powerful relic of the Kil'jaeden. [For more power]. Losing so much of their army at such a critical point, they failed at Lordaeron, and attacked and murdered the betrayers. The Horde failed in their war. Many years passed, and the Legion was coming back for the Orcs. [Remember, they were corrupted by the Kil'jaeden, but not all of the Orc clans were corrupted, and those that were not corrupted or broke from the Legion escaped to Kalimdor under Thrall's command... That is a basic run down, the Orcs were Corrupted and later on with the Legion the Humans also betrayed an alliance too fight the Legion... That is a run down simply, for more go too... www.wowwiki.com