The Broken Front is the sub-region of Icecrown located just south of Mord'Rethar: The Death Gate. Many daily quests from The Skybreaker and Orgrim's Hammer require you to come here to slay Scourge forces.
Described by these quests:
The Alliance set into motion an offensive against the Scourge forces at the gate of The Broken Front, with the intent to steal a march on the Horde and take possession of the gate. The operation was planned in secrecy, and would likely have succeeded but for a chance encounter with a Horde recon patrol.
On spotting the Alliance forces, the Horde patrol took cover. Once the alliance offensive was fully engaged with the Scourge, the patrol attacked from behind. While the patrol was considerably weaker than the Alliance forces, the surprise of the attack was enough to break up the alliance formation, inflicting grievous losses.
Argent Crusade forces arrived late to the battle after all three sides had mutually annihilated each other. They were able to retrieve a few members of both the Alliance and Horde forces back to Justice Keep, but the majority of the gravely wounded remained where they fell.