That depends though. In Wc2, it follows the Human campaign storyline. However, on the Orc side, you can turn the tides by eventually leading to the eradication of tLordaeron Castle.
By all accounts if Gul'Dan hadn't betrayed Doomhammer, and taken a good deal of the ORcs power with him, Both sides knew that any attack on Lordaeran would have given Orcs a brutal victory, aided in no small part by the fact the ORcs had already sacked Qul'Thalas and Khaz Modan (if I got the Elf and Dwarf city names wrong, sue me for not remembering a champaign I played a decade ago).
Think about the Undead now, had to invade Lordaeran all sneaky and ****. The Humans fell apart in no small part simply because Arthas assassinated his father and dismantled any sort of authority. Lordaeran wasn't at war, and fell apart becuase it wasn't prepared. However the ORcs and Humans were at fullscale war, and the Humans still lost. Both Evil sides conquered lordaeran, However the way Orcs did it was the same reason they could take the undead.
And I say the Orcs conquered Lordaeran with the realization that they only lost because the Orcs fought with the Orcs, and gave the humans a small chance which they exploited. Also Remember the undead are ruled by Orcs. The simple fact is Ner'zul was an Orc, and he commands the Undead. The Liches were Orc warlocks. The Original Horde had the spells of the Undead, they had the Deathknights to. Orcs were clearly a stronger force.