Speak to Voggah Deathgrip at Alterac Mountains in Hillsbrad Foothills.
I will study the Stormpike battle plans and prepare for an assault. I need you to go to Alterac Valley and enlist the aid of the Frostwolf Clan: the very clan of our former warchief, Thrall. They will help us battle the Stormpike dwarves! Together, we will crush the invaders and see them driven out of Lordaeron for good!
Go now, <name>. Alterac Valley is northeast of Tarren Mill. Find Voggah Deathgrip and tell him that the Horde requires the aid of the Frostwolf!