"The Warchief has issued the Uncrowned their highest command. This mission is exceedingly dangerous: infiltrate Stormwind and rescue Prince Anduin Wrynn, who has been placed under house arrest." The half-orc Garona revealed the supreme mission Rend had assigned to the Uncrowned.
After the Alliance High King Varian Wrynn returned to Stormwind, he issued an emergency order recalling every legion to Stormwind, including Marshal Windsor's Second Legion in the Burning Steppes.
However, Marshal Windsor noticed something strange about his High King. Rather than obey the order, he chose to remain stationed in the barren Burning Steppes.
Furious at Marshal Windsor for ignoring his command, Alliance High King Varian Wrynn ordered Lakeshire in the Redridge Mountains to cut off the Second Legion's supply lines.
Rather than making Marshal Windsor back down, the severing of those supply lines only strengthened his resolve. Their High King had definitely gone wrong somehow, so they continued holding their ground in the barren Burning Steppes.
Because the Horde army was too massive, it would be split into two forces, one landing in the north and one in the south of the southern Eastern Kingdoms. The southern Horde force had already dealt with Duskwood and Westfall, the barriers to attacking Elwynn Forest. The northern Horde force, meanwhile, had to pass through Dwarven territory, the Burning Steppes, and the Redridge Mountains.
With Muradin Bronzebeard persuading the Dwarves, there should be no problem there.
Only Marshal Windsor's Second Legion in the Burning Steppes had become the greatest obstacle. Marshal Windsor could do as Rend had done before and blow up Blackrock Spire's stone bridge, preventing the Horde army from crossing the lava lake.
Marshal Windsor certainly would not aid Varian Wrynn, who was controlled by Y'Shaarj, but neither would he allow the Horde army to pass through the Burning Steppes