"from the Pillars of Hercules here in the west to the waters of Egypt and Syria here in the east"

The Pillars of Hercules was the ancient term for the strait of Gibraltar (specifically the land on either side of the narrow stretch of sea leading out to the Atlantic Ocean). So, from the Pillars of Hercules to Egypt and Syria, Pompey basically means the entire Mediterranean Sea.

The Pillars of Hercules
Permission Granted by Copyright Owner for Use on Book DrumThe Pillars of Hercules - Credit: John Eckert