With the death of Megas Alexandros in 323 BC and no immediate heir to his vast empire, the lands from Hellas to Arachosia were divided between his senior officers.  These men became known as the Diadochoi, or Successors, and in the twenty-two years between then and the Battle of Ipsos their numbers dwindled to five: Antigonos Monophthalmos in Anatolia, Kassandros in Makedonia, Lysimachos in Thraike, Ptolemaios Soter in Aigyptos, and Seleukos Nikator in Babylonia.  Of these five only Antigonos, in his central position surrounded by the other Diadochoi, was ambitious enough and aggressive enough to overtly attempt to conquer all of what was once Alexandros's empire.
With his son Demetrios Poliorketes, Antigonos made continual strides in extending his kingdom at the cost of the other Diadochoi.  With only a few exceptions he had continual victories over his opponents – victories which compelled Antigonos to crown himself and his son as kings in 306 BC after conquering Kypros.  In response to these actions Kassandros, Lysimachos, Ptolemaios, and Seleukos crowned themselves as kings and formed an alliance to combat Antigonos.  Thus, in 301 BC near the city of Ipsos in Phrygia on a plain in front of the valley that is now known as Hammam Gazigöl, the forces of the allied Diadochoi under the command of Seleukos fought against Antigonos and his son.
The specifics of the battle are unclear as only two accounts are extant that describe the battle.  The first is in Plutarch' biography of Demetrios, which mainly focuses on the heroic character of and the events as they affect Demetrios during the battle.  The second comes from the foremost historian of the era Diodoros Sikeliotes, but unfortunately his account only survives in small fragments.  What can be reconstructed, however, is as follows:
The forces of Antigonos numbered 70,000 infantry, 10,000 cavalry, and 75 elephants.  Against these Lysimachos and Kassandros brought a majority of the allied infantry, which was augmented by 20,000 light troops, elephants, chariots, and cavalry brought by Seleukos for a total of 64,000 infantry, 10,500 cavalry, an astounding 400 elephants, and 120 scythed chariots.  The dispositions for both armies were probably similar with the phalanx in the center and guarded on the wings by cavalry; the elephants were in front of the main line and accompanied by light troops (but whereas Antigonos only had a fraction of Seleukos' corps, the latter kept several hundred in reserve).  Both Antigonos and Seleukos were in command of the infantry in the center.  The majority of the Antigonid cavalry was under command of Demetrios on the right wing.  The Seleukid left and right wings of cavalry were commanded by Antiochos and Lysimachos respectively – the former of the two was the son of Seleukos and future king of the Seleukid Empire.
The battle opened with Demetrios charging headlong at Antiochos, who retreated down the valley of Hammam Gazigöl with Demetrios in hot pursuit.  And, now having this opportunity to remove the majority of the Antigonid cavalry from the battle, Seleukos sprung his trap and moved his elephant reserve to block the pass of Hammam Gazigöl while the remaining countered the 75 of Antigonos.  Demetrios became isolated from his father and the rest of the Antigonid army was without its elephants.  The main battle lines now closed with each other.
Although the Antigonid phalanx was of more men and deeper, it was not enough to gain victory.  Because Antigonos' right flank was completely unguarded Seleukos was able to easily slip his light troops and Dahae cavalry behind the opposing phalanx.  Through calm use of missile fire the Antigonid troops fell apart and either defected or routed.  In the end, Antigonos himself died being pierced by a javelin.
The battle was won and the remaining Diadochoi were victorious.  There were, however, three major consequences of the Battle of Ipsos.
The first and most immediate was that the Diadochoi of the alliance agreed that each would rule his respective kingdom independently of the others and that none could hold kingship over another  This is widely cited as ending the last chance of reuniting the empire of Alexandros; however, that hardly prevented Seleukos from reclaiming the majority of it until his assassination in 281 BC by Ptolemaios Soter's spurned, elder son Keraunos.  The second major consequence was that the territories formerly held by Antigonos were divided between the participants of the alliance, which leads directly into the third consequence.
The lands of Phoinike and Koile-Syria were granted to Seleukos at the conclusion of Ipsos.  These lands had previously been owned by Ptolemaios, but lost to Antigonos in 315 BC.  Although he participated in the alliance it was largely passive as he was neither present at the battle nor did he send any troops.  When Seleukos arrived to assess his new territories he found Ptolemaios already there.  Seleukos was hardly pleased, but did not press the issue out of “friendship”, even though he likely could have taken the lands by force.  Regardless, it would prove to be a sore point between the two kingdoms and partially responsible for the spawning of six wars between 274 and 168 BC, which would later become known as The Syrian Wars.
As to what happened to Demetrios, after the battle he escaped to Ephesos with 5,000 infantry and 4,000 cavalry.  He left for Greece and eventually managed to seize control of Makedonia away from the family of Kassandros and continued the Antigonid dynasty.  This then lasted until the events of the 3rd Makedonian War when Perseus lost The Battle of Pydna against the Romans in 168 BC.
There is a final note of interest.  Despite having brought 120 chariots to the battle and the relative flatness of the terrain, it is likely that Seleukos did not utilize them.  Perhaps he realized their ineffectiveness or their risk of becoming uncontrollable just before the beginning of the battle.  If Antiochos III Megas has followed his ancestor's example he might not have lost the Battle of Magnesia over a century later when his entire left wing fell apart due to rogue chariots.

Playing the Battle:
You are in control of the allied forces and before you is Antigonos' army.  As the attacker you must go to him for he has the advantage of men and is defending his territories.