The Battle of Pharsalus. It is 48 BC. Julius Ceaser has conquered Gual and made play to become the head of the Roman State. The political divisions in Rome are deep, however, and Ceaser has rivals. Pompey the Great is chief amoung them. Pompey, much like Ceaser, has made a name for himself by driving the vastly superior Roman legions against border states in the East. He has expanded the Empire, like Ceaser in Gaul, and has just as legitimate claim on power. Pompey is supported by his allies in the Roman Senate and by his conquered vassel states.
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Keep in mind that this battle is fought between essentially equally matched forces, technologically speaking. Both sides are making use of the legion, which is the dominant heavy infantry war machine of it's day. Ceaser's troops are veterens of his campaigns in Gual. Pompey outnumbers Ceaser and employs Greek and Syrian troops to supplement his ranks.
Overall, the advantage is to Pompey. Pompey has about 40,000 men and Ceaser only has 22,000. In addition to numbers, Pompey has a vastly superior cavalry wing. The weakness in Pompey's forces, however, is that they are largely inexperienced. This will benefit Ceaser in several ways.
Pompey was actually camped on the side of a mountain called Dogantzes some miles from Pharsalus proper. Ceaser was actually positioned closer to the city several miles across a plain from Pompey.
This is where several key aspects of Ceaser's genious and experience come into play. Pompey had massed his forces with his right protected by a river and the superior cavalry forces protecting his left. To open battle, Ceaser knows that he must cross the plain in a charge, which, over several miles, will exhaust his men. Ceaser anticipates that Pompey will order his cavalry to flank him and try to crush his legions in a vice. To counter this, Ceaser makes a fourth battle line behind is cavalry on his right.
In the middle of the charge, Ceaser stops. This confuses Pompey's lines, who are all rowled up expecting to break an infantry charge. Instead Ceaser closes the field from here in a steady march. This causes Pompey to order his cavalry to attack Ceaser's right. The lines meet in the center and legion to legion are locked in a stalemate. Ceaser's lighter cavalry breaks and is routed, as anticipated. However, Ceasers fourth battle line attacks the cavalry at this point and Pompey's cavalry never makes the anticipated charge into Ceaser's flank. Unable to keep up a defensive action against a legion with their momentum broken, Pompey's cavalry retreats into the foothills to try and regroup.
At this point, the Ceaser's fourth line executes a left wheel into Pompey's rear and left flank. It's all over. Pompey's army is out maneuvered. Magnanomously, Ceaser orders his army to only kill the troops from Pompey's vassel lands and not their own coutrymen.
The Roman Senators in Pompey's camp are put to the sword. Ceaser takes command of Pompey's remaining Roman troops and gives them a pardon. Pompey's ally troops are slaughted. Pompey himself makes it to the sea and escapes to Eygpt.
Ceaser later hears Pompey is in Eygpt and pursues him there. In Eygpt, Ceaser meets Cleopatra and fights the first battle of the Nile. But, that is another story.