Speak to Packmaster Stonebruiser.
We will never control the Eastern Plaguelands if we do not control Stratholme. Not only is it a point of incredible strategic importance, it also houses our enemies' leaders - Balnazzar and Lord Aurius Rivendare. Their Risen and Scourge armies threaten to erase all the progress we've made.
The Argent Crusade is well-suited to this task. Travel west through Stratholme's main gate and speak with Packmaster Stonebruiser. He will direct you further.