So, This has probably been said a lot, and I'll bet I'll get a lot of "Welcome to the party" Type comments, but, here goes.
I've been thinking about the events that the infinite tried to prevent. And I saw a common theme between them all. All of them would have prevented the birth of the New Horde. Thrall fails to escape Hillbrad or dies there? Bam! No new Horde. Medivh doesn't open The Dark Portal? Boom! Second War never happens, orcs never come back to Azeroth, and new horde. The Infinites don't show up in Mount Hyjal, so that doesn't count. But what about Stratholme? Arthas dies before he can fight Mal'ganis, what happens? Well, for one thing, Arthas dying during the events of The Culling mean that He's now dead. No lich king. Which leads to no Silvanis. The Blood Elves are never formed either. How ever, with Arthas dead, his order that the Silver Hand disband never happens, or rather, it happens, but it doesn't have to be followed. Also, remember that since we were there, they found the plagued grain. This leads me to believe that if we weren't there, they wouldn't have found crates. This leads to greater amounts of dead, but it also means that the Alliance bands together stronger. Maybe the events leading up to the present of WoW play differently. But the point is that, with no Lich King, two of the 5 groups of the Horde are removed. The Forsaken aren't formed because...well, Duh. And the Blood Elves never become such. So. Less powerful horde=More Powerful Alliance, equals no new horde as such.
So, why are the infinite trying to erase the horde? Thoughts?
Edit: Was talking with my friends and some other timeline changes occured to me
Culling of stratholme: Arthas never goes to northrend, which causes the alliance to not lose Muradin. Also, Arthas never kills his father, leaving Lordaeron in alliance hands for the third war.