When Arthas led the Undead Army against the Kingdom of Gilneas, not a single nation in the Human Alliance lifted a finger to help. Yu Lin Bloodhawk believed this was largely retaliation for Gilneas selfishly withdrawing from the Alliance during the Second War, mixed with pleasure at its misfortune. That was why the Alliance nations had all independently chosen to stand by and watch.
When Arthas led the Undead Army against the High Elf kingdom of Quel'Thalas, the nations of the Human Alliance once again stood aside. The fundamental reason was not that Quel'Thalas had withdrawn from the Alliance after the Second War.
Quel'Thalas's withdrawal after the Second War was merely an excuse the Human Alliance nations used to justify their own inaction. Such a pathetic excuse was really nothing more than a flimsy fig leaf.
The deepest reason they refused to help was that humans had never cared whether the High Elves lived or died. One could even say that they would rather see every last High Elf dead.
This was plain to see from what happened after the destruction of the Sunwell in Quel'Thalas, when Kael'thas led the Blood Elves into the Alliance to fight the undead, only to be persecuted by the Alliance's Grand Marshal Garithos.
This could not simply be dismissed as Grand Marshal Garithos acting on his own, because he lacked the power to bear responsibility for such a decision alone. It could only be seen as a murderous act collectively agreed upon by the leaders of the Alliance nations, kicking others while they were down with the intent to kill them.
They not only wanted to use the undead to wipe out the High Elves' last surviving elite troops, but also imprisoned Prince Kael'thas in Dalaran and prepared to sentence him, cutting off any