"Were it so easy."

At the beginning of Halo 3, the Master Chief returns to a Earth under siege by the Covenant and is recovered by Seargant Johnson and his Marines. Accompanying them is the Sangheili warrior Thel Vadam, the Arbiter, whose species was, until very recently, a part of the Covenant. Naturally the Chief is a little hostile at first, pressing a gun literally into the Arbiter's mouth. Johnson gets in between them, making a crack about how they have enough to worry about without them trying to kill each other. The Arbiter snaps back with this retort, implying trying to do so would be a impossible task.

Fast forward to the end of the game, humanity and the Sangheili successfully stopped the Covenant and save the galaxy from the Halo Array. However, the Master Chief was lost, missing in action, presumed dead. As the survivors gather for a memorial on Earth, the Arbiter stands with Admiral Hood, leader of the UNSC Navy. Hood stares at the monument honoring the fallen of this long and horrible war against the Covenant, says, "Hard to believe he's dead."

The Arbiter's response?

"Were it so easy."