Time Traveler

Suddenly I am in an unfamiliar land. A hundred meters away a battle is underway. I am behind Caesar's legions, with Pompey's on the far side. Caesar's forces are badly outnumbered here. It is clear they will be overwhelmed in a matter of minutes. A legionnaire comes running past me, oblivious to my presence. He carries something in his hand. He is running toward the centurion who is obviously the leader, a message perhaps. One of Pompey's soldiers breaks through the line and attacks the messenger, slaying him, then moves on to attack others. I run to the messenger's body and retrieve the message. It contains a single word:

PHULGLDQXV

I'm a linguist, so I know know Latin, but this is not a word in any language. I have long enjoyed solving cryptograms and other cipher puzzles, so perhaps I can help. Of course! Caesar used a simple cipher. Now how did that work? But I have no time. Pompey's men have broken through the line. I must flee immediately.