"Do you have plans for the Kingdom of Gilneas?" Sylvanas did not know what Patrick intended. But when Arthas invaded, Sylvanas had ignored advice and trusted the Five Great Families too much, leaving her waiting in vain for reinforcements from Silvermoon City at the entrance to Bann'donor. In the end, she had been defeated after a bitter struggle and killed by Arthas.
Although Sylvanas had discovered a traitor within the Scourge, he had not expected that traitor to be a senior member of the Council. Moreover, that traitor had secretly infiltrated the Isle of Quel'Danas and disabled the Bann'donor Energy Field, resulting in the complete annihilation of Sylvanas's Farstriders and the Bann'donor garrison.
Now, with her own experience serving as a warning, Sylvanas chose to trust Patrick.
"I think Gilneas should serve my purposes. Even if they don't join the Arcane Pact, we can't let them join the Alliance."
On Azeroth, the Alliance was the most powerful force. The Night Elves in particular were unquestionably the strongest race in the world. Moreover, after the Draenei joined, the boost from advanced new technology would undoubtedly send the Alliance's strength soaring. Patrick could not allow the Kingdom of Gilneas to join the Alliance and further increase its strength.
If left unchecked, the Alliance of the future would probably require the Covenant and the Horde to join forces to contend with it. So, first, he absolutely could not let the newly formed Avengers push Gilneas into the opposing camp. Second, he had to bring the Kingdom of Gilneas into the Arcane Pact. Even if he could not recruit them, he had to eliminate any possibility of their joining the Alliance.
Although the Alliance of Lordaeron and the Kingdom of Azeroth Alliance were both alliances, there was a vast difference between them, and their