The Alliance has zero reason to start conflict, however. Especially now, after seeing just how futile the war actually is. We’ve all seen what happens in the afterlife, we know we pretty much all go to the same spots regardless of faction, so that knocks a ton of wind out of the sails of faction conflict. At this point, the only reason to start up the Fifth War is because of insane bloodlust. Alliance and Horde internal security is most likely extremely sensitive to any attempts at infiltration by extremists or agent provocateurs that would set their faction on the path to war.
There’s literally less-then-zero reason to go to war with each other. Spats between individuals? Maybe, but there is absolutely no reason for the Alliance and Horde war machines to move against each other.