Derthelz on the official forums asked a good question -- why is the Alliance prepared to kill Arthas but not to try to retake their lost city of Lordaeron? After all, the upper half of the Eastern Kingdoms is almost completely lost to the Alliance, despite the area being the former home of some of Azeroth's most powerful nations. Why doesn't the Alliance try to take them back?
Crygil says that, actually, they just might:
"Who said that the Alliance wouldn't attempt to reclaim these lands? That is a very dangerous assumption to make, especially if you're of the forsaken persuasion.
The Lich King is a threat to all the inhabitants of Azeroth and, as such, he needs to be put down before territorial disputes can be resolved. Leaving those claims for the future only makes sense; far easier to fight one war at a time. For now, it's better to focus on the task at hand."