It seemed like a pretty good plan: Go to a version of Draenor where the orcs had not yet become slaves of the Legion, and instead rally them independantly to invade Azeroth.
However, the Iron Horde did not even get past the Blasted Lands, and after spending so much time building their precious portal, it was blown to pieces in no time at all. A dream team of Khadgar, Thrall, Maraad and other champions immediately stood ready to push the Iron Horde right back where they came from. The moment the Dark Portal collapsed, the plan was basically ruined.
How could such a huge, organized army have been foiled so easily by a handful of people? What should the Iron Horde have done differently in order to succeed?