Lilkitten: The Alliance definitely betrayed the blood elves. The entire blood elf campaign in WC3 was tailored to show how they were systematically disenfranchised by the Alliance and the incredibly racist Garithos. The remaining high elves account for, as I recall, 1/100th of their original population. The rest were either killed during the invasion or became blood elves afterwards.

Also, your facts regarding the Forsaken are simply wrong. You're confusing them with the Scourge. Arthas, Prince of Lordaeron, slaughetered his own people, and THOSE are the Forsaken. They're the citizens of Lordaeron who were killed by their King but regained their free will. They were subsequently deemed monsters by the rest of the Alliance and joined the Horde.

The old Lordaeron / Silvermoon alliance between humans and high elves lives on in present WoW as the alliance between the Forsaken and blood elves. This was even confirmed with in-game text when BC came out, and the Forsaken commoners were saying things like, "Ah, fighting alongside the elves again, just like old times!"