Thrall gathered a renegade group of lethargic Fel Orcs and purged them of any demonic magic. Proclaiming that they should be free of such influence forever, he is poised on freeing his brethren that are held captive in Durnholde. Transport ships are waiting to take them all to Kalimdor. He has also allied several troll tribes.
The Horde, lstarts scattered. You should take advantage of that, and creep extensibly as far as possible before being forced to flee. Lordaeron has a huge number of your kind kept prisoners in Durnhold; if you manage to break through their fortress, you can free them and get yourself an incredible boost. Still, your biggest chance lays in getting your people into ships and reaching Kalimdor as fast as possible.
Warsong Clan - They start as scattered as you. You both share a settlement on the south end of Eastern Kingdoms, and your initial Kalimdor settlements are quite close. You should move to help them as fast as possible. You need to set up a strong presence in central kalimdor to check the expanding Night Elves.
Bloodpact - When the Legion is summoned, and if Mannoroth is still alive, Teal can offer you the Blood Pact. Should you accept it, you will lose all existing units/buildings/territories, whose ownerships will be transferred to an angry grey. You and grey will automatically unally as well. Grom Hellscream and a number of fel orcs will respawn in the Twisting Nether. This is a betrayal event, one that will help an already winning Evil team and surely let you former ally be overwhelmed by Night Elves.