As for Sylvanas, the current Banshee Queen was something of an outsider in the Horde. The Forsaken seemed to have nothing to do besides seeking revenge against the Scourge and fighting bitterly with the Scarlet Crusade, and Thrall seemed to turn a blind eye to Sylvanas's actions.
After order was restored in Quel'Thalas, Sylvanas's connections found the Horde a good partner, temporarily relieving its shortage of supplies. From then on, Thrall gave Sylvanas even freer rein.
Politically speaking, Thrall truly did not dare offend Sylvanas too much. All the Horde's forces in the Eastern Kingdoms depended on Undercity alone to relay intelligence and rotate soldiers. If they lost Undercity, the Horde would lose its say throughout the Eastern Kingdoms.
Originally, Thrall had wanted to use the Alliance to eliminate Kael'thas, then have a devastated Quel'Thalas join the Horde. With Silvermoon City and Undercity supporting each other from afar, the entire northern end of the Eastern Kingdoms would become Horde territory.
Thrall's thinking was entirely sound. Quel'Thalas had already fallen out with Dalaran and had no fondness for the Alliance either. If it lost its army in Outland, Quel'Thalas, its last hope dashed, could only choose to join the Horde and settle for holding on to its corner of the world.
But what Thrall had not expected was that Quel'Thalas had made its arrangements long ago. Back in Outland, after seeing Patrick hurry back to Azeroth with a large army, Thrall had kept speculating that Patrick might use the Alliance to eliminate Kael'thas and then crown himself king. That idea had been wrong. Now Kael'thas had returned to Quel'Thalas with the Sunfury, Eclipse, and the Fire/Demon Seal troops.
So, to Thrall, Undercity would be the Horde's voice in the Eastern Kingdoms.
How could Patrick be unaware of Thrall's little maneuvers? Since