The Dwarf Garrison and Southshore were both taken out , before South shore was the main supplier of all Alliance forces in Lordaeron There is no proof the Forsaken started the conflict in Hillsbarad. Varian still had a desire to take Lordaeron a desire he held onto until Sylvanas beat it out of him in Cata. The Dalarn mages also planned to aid the Alliance in taking over Lordaeron in Silverpine.
on the plague, it was used without the approval of Sylvanas or most of the Forsaken, it was done by Putress and Varimathras.
The Alliance got itself into a war with the Forsaken, and now the Forsaken have to build themselves up to continue said war.