Antiochos III returned from his anabasis as Antiochos Megas and Basileus Megas.  Seleukid rule once again extended from East to West.  He now set his attentions to managing Mikra Asia – a region that had always been difficult to control, due both to its inhabitants and to the recurring perfidious nature of its appointed governors.  However, once word had reached him that Ptolemaios IV had died, Antiochos directed his attentions once more to conquering Phoinike and Koile-Syria.
The peace treaty that ended the 4th Syrian War after the Battle of Raphia, like most Hellenistic treaties, existed only as long as the signers were alive.  To put it in a more cynical fashion, as soon as one of the signers was dead, the treaty was null.  Sosibios (the same minister who organized the military during previous war) and Agathokles, the two chief ministers of the Ptolemaic kingdom at this time, were well aware of this and immediately after the death of Ptolemaios IV in 204 BC set in action a series of measures to counter any attacks by Antiochos.  Among these measures the first was to keep the death of the king secret so as to allow the government of the child king Ptolemaios V time to prepare for war (although the transition between governments was most likely smooth as even during the reign of Ptolemaios IV his ministers had done everything).  By August/September 203 BC, knowledge of the death became public with the crowning of Ptolemaios V and the regency government sent out three high profile diplomats.
The first of these diplomats was sent to Antiochos requesting that he continue to honor the treaty of 217 that ended the previous war.  He was, of course, rebuked.  The second diplomat was sent to Philippos V of Makedonia is an attempt to gain an alliance through marriage, and thus prevent Antiochos from attacking with a potential enemy at his back.  This diplomat also was rebuked.
The third diplomat was originally intended to make his way to Rome through Greece; however, considering Rome's current situation with fighting against Hannibal Barca it is unlikely that the diplomat ever reached his destination.  Regardless, this third diplomat was more successful and instead spent his time vigorously re-establishing Ptolemaic contacts throughout the region and hiring mercenaries.  The most prodigious of these was the Aitolian League – a confederation of city-states centered around the region of Aitolia with a significant military power.  From the league the prominent statesman Skopas was recruited, who brought with him 6,000 Aitolian hoplites to Alexandreia.  Antiochos, not one to wait, was already well underway with his own preparations.
Besides preparations for his military to again set out on campaign, Antiochos wanted to guarantee the security of Mikra Asia.  The region would still need years of stability before it settled down, but if Antiochos was occupied with the royal army fighting in Syria even a small disturbance could destabilize the entire peninsula.  For this reason he contacted Philippos in the hopes of reaching an agreement.  This agreement, which was supposedly only known amongst the two kings, became known as the “secret alliance”.
The actual terms of the agreement are unknown, but whenever it has been described by the historians of the era it garners an unsavory characterization.  Generally, it has been stated to be a partitioning of the Ptolemaic kingdom by Philippos and Antiochos; however, the actual results of history go against this.  Consequently, some modern historians doubt such an alliance even existed.  In fact, the alliance, if there ever was one, seems to have only been a ploy for Antiochos to keep his holdings in Mikra Asia safe from Philippos' predations.  By persuading Philippos to attack Ptolemaic holdings in the Aegean, Antiochos was able to further strain the Ptolemaic military, but also to ensure that Philippos was occupied.
Not surprisingly, Philippos proceeded to make a complete fool of himself.  He attacked not only Ptolemaic holdings, but also several independent Greek cities, of which some were allied to Antiochos and others to Rome.  The results were that several of these Greek cities appealed to Rome for help, which was the cause for the 2nd Macedonian War, and the alliance between Philippos and Antiochos, if there ever was one, was broken.  Ironically, by the time the alliance, real or imagined, became public knowledge it seems to have been long gone.  From then on relations between the two Makedonian kings remained cool at best.  Regardless, Antiochos found his holdings for the most part unperturbed, which left him free to concentrate on the war with Ptolemaios.
In 202 BC the 5th Syrian War began with the invasion of Koile-Syria by Antiochos.  Again, as he had done years earlier, Antiochos was successful and occupied Ptolemaic territory as far south as Gaza.  This heavily fortified city was the only thing stopping Antiochos, but his campaign had been lengthy and he decided to spend the winter of 201-200 in Syria saving Gaza for later after leaving troops to garrison specific locations.  Even before the beginning of the campaigning season of 200, the Ptolemaic counter-attack began.
Skopas, the recruited Aitolian general, was placed in command of the Ptolemaic army charged with retaking Koile-Syria.  And while Antiochos was still away in Syria he led his army and reoccupied the region in the winter of 200.  As soon as Antiochos heard of the attack he led his troops south to meet the Ptolemaic army.  The two armies joined in battle in the summer at a small village on top of the Golan Heights named Panion.
The geography of the land surrounding Panion is complex.  The village rests on what is now known as the Banyas Plateau.  The plateau is split into a northern and southern half by a ravine.  The northern half is flat with a gentle gradient rising toward the height known as Tel el-Hamra.  This small mountain is dominated by Mt. Hermon behind it.  The southern half is rough, uneven ground that extends to the west and ends with Tel 'Azzaziyat and to the south with Tel Fakhr.  It is on the southern half that Panion sits at the base of another rise to the east.
In the early morning Antiochos took the initiative and took control of Tel el-Hamra and Tel Fakhr with his cavalry and light troops leaving Skopas to react and fight at a disadvantage.  The disposition of Antiochos' troops was possibly as follows: 
In the northern arena under the command of Antiochos Megas himself were, from right to left, the hetairoi, the hypaspistai, and then the argyraspides, citizen phalanx, and Tarentine cavalrymen; in front of the phalanx were the elephants, missile troops; finally, high up on Tel el-Hamra were the kataphraktoi under the command of Megas' youngest son Antiochos the Younger – the future king Antiochos IV Epiphanes.  In the south, the remaining half of the army was under the command of Megas' eldest son Antiochos the Elder.  Again, from right to left, were the cavalry agema, thorakitai, and thureophoroi; behind the infantry were elephants forming a second line; to the left of these troops were cavalry and light troops stationed diagonally on Tel Fakhr.
Skopas realized that he was at a disadvantage because of his position on the Banyas plateau.  Because of this he may have planned to capture the Seleukid aposkeue – the baggage and dependents, which were based in Panion.  If he could capture or at least threaten these, it might cause the collapse of the entire Seleukid force as it rushed to defend its belongings and rear.  For this effort he deployed all his heavy infantry, including the Aitolian hoplites, in the southern arena just below Tel 'Azzaziyat supported by about half his cavalry placed to the right of the line.  And if capturing the aposkeue could not cause the desired effect, then those troops could swing around and take the Seleukid phalanx in the rear.  Skopas, however, was in the north with his phalanx on the level ground.  To the left of the phalanx was Aitolian cavalry and to the front were the elephants and missile troops.
The fighting in both arenas likely began at the same time.  In the north the elephants engaged each other as the lines closed.  The outcome was predictable as the Indian elephants of the Seleukids with the help of the missiles and Tarentines easily drove off the smaller Africans and missiles of the Ptolemaioi.  With the lines closing the Seleukid phalanx likely made lanes for the elephants, skirmishers, and cavalry to pass through to a rearward position.  Then the Seleukid kataphraktoi, possibly having been previously concealed on an unseen ledge now gone after 2,200 years of erosion, came charging down Tel el-Hamra.  
The kataphraktoi struck the Aitolian cavalry in the flank and sent them into a complete rout.  Without their cavalry support to guard their rear, the Ptolemaic phalanx was in a dire position.  Skopas, having seen that the kataphraktoi were devastating his ranks and that this position was hopeless, rushed to his troops in the south hoping to salvage victory.  Thus, the battle was won in the north by the kataphraktoi taking the phalanx in the flank.
The Ptolemaic force in the southern arena was showing itself to be more successful than the now destroyed phalanx in the north.  The Seleukid line here was hard pressed and began losing ground to the Ptolemaic line.  It is likely that there was a short break between fighting (something which occurred often between bursts of fierce combat), which allowed the Seleukid line to retreat behind the elephants.  The elephants then charged and scattered the Ptolemaic infantry and cavalry.  Skopas, for all his luck, might have made it across the ravine just in time to see the last of his army melt away, or he decided that the situation was hopeless and ordered a withdrawal with what few men were left.
All that Skopas managed to salvage from Panion was approximately 10,000 survivors.  With these men he retreated northwest to Sidon in hopes of being evacuated by sea.  Antiochos quickly followed after him and besieged the city.  The Ptolemaic navy failed to arrive and Skopas was forced to surrender to Antiochos.  Koile-Syria was now in Seleukid control and the Ptolemaic kingdom could not afford to send another army to contest it.  The 5th Syrian War, however, would continue for another five years.
Antiochos moved south and laid siege to Gaza.  The fortress-city fell in 198 and Seleukid control extended further south all the way to Raphia, the site of Antiochos' loss in 217, which ended the 4th Syrian War.  Aigyptos was now open to invasion, but Antiochos had different plans for the region, which was still embroiled in a major rebellion by the native Aigytioi.  A campaign to seize just Alexandreia itself could  take years with little actual return in investment.  Furthermore, there was a large Ptolemaic army consisting of klerouchikoi, including the Galatikoi, stationed in Lykopolis in the Delta in order to combat the growing rebellion.  This army was in a position to easily contest any crossing of his forces across the Neilos.  Instead, he looked to the Ptolemaic territory in Mikra Asia, which not only was much more enticing, but also in keeping with his goal of restoring the entirety of the Arche when it had been at its greatest extent under the reign of its founder Seleukos I Nikator.
As soon as the passes opened in 197 Antiochos went on campaign in Mikra Asia.  Throughout the spring and summer Antiochos pushed the Ptolemaioi from their holdings on the southern and western coast of the peninsula.  By 196 he had completed his objectives and focused his energy on projecting his power into Thraike.  Ptolemaic power was now restricted to Aigyptos.  Finally, in 195 terms were agreed upon and a treaty set into place cemented by a political wedding.  The ceremony took place at Raphia a few years later with Antiochos marrying his daughter Kleopatra to Ptolemaios V.  
The marriage was the final step in the ending of the 5th Syrian War.  Antiochos probably hoped that the marriage would secure his new territory for his descendants after his death since he was much older than the young Ptolemaios, who was only fourteen or fifteen years of age.  Over the next few years Antiochos kept himself occupied in the western reaches of his empire gradually expanding his power.  His interaction with the Hellenic city-states, which would prove critical in the near future, brought mixed reactions.  Antiochos was mostly accommodating, cleaning up the mess left behind by Philippos V's campaign years earlier.  He even restored the destroyed city of Lysimacheia and placed his son Seleukos (the future king Seleukos IV Philopator) as Strategos of Thraike.  Unfortunately, his actions would in the future provoke derision from Rome, which was extremely paranoid after the experience of Hannibal.  For now though, Antiochos III Megas could be considered restitutor orbis: restorer of the world.  Upon his return from India he earned the epithet "the Great".  Now he earned the title Great King.

Playing the Battle:
This battle should generally be considered a multiplayer battle.  The Seleukid player has the terrain advantage, but the Ptolemaic player has a numbers advantage.  If you as an EB player wish to take a look at the battle by yourself, the singleplayer option remains.  Be warned though: the AI has interesting behavior.