This is set in the Wars of the Diadochi (successors) which were a series of conflicts following the death of Alexander as his generals tried to either conquer what Alexander had or create their empires. The most successful was Antigonos the One-Eyed who would at time own everything but Egypt, Macedon, and Thrace. However recent setbacks including Seleucus regaining Persia, Babylonia, and the other eastern parts left Antigonos with Anatolia, the Levant and parts of Greece. He would focus on the West and his army led by his son Demetrius would repeatedly defeat Cassander in Macedon and almost took over the whole thing but Cassander would ally with Lysimachus who owned Thrace, Ptolemy who owned Egypt, and Seleucus who was currently marching west after fighting a war with the Mauryan Empire of India in which he had to give vast swathes of land in exchange for 500 war elephants. This coalition would see Lysimachus land in Anatolia and do much damage and the 81 year old Antigonos would march out while having his son and the army under his command to meet him and defeat Lysimachus. Cassander, given a brief respite, would send reinforcements to Lysimachus but out of the first batch, the other two would either be sunk by the vast Antigonid navy who was based near the Hellespont or in a storm. The two sides would play a cat and mouse game as Antigonos tried to defeat the coalition while the coalition wanted to meet up with Seleucus who was nearby having marched from India to Anatolia in about a year. Ptolemy would attack north and take over the Southern Levant.
301 BC: Antigonos and his son Demetrius the Besieger are pitted against a coalition of their rivals, Lysimachus, Cassander, and Seleucus near the village of Ipsus, in Central Anatolia. Both sides have massive armies with the Antigonids having 80 thousand troops (70 thousand infantry and 10 thousand cavalry) and the coalition which had 79 thousand troops (64 thousand infantry and 15 cavalry and 500 elephants). The battle started when Demetrius and the 8 thousand cavalry under his command routs the forces on the coalitions left.
POD: Demetrius instead of pursuing, moves back to shore up the Antigonid flanks.
The two sides would engage fully with the Antigonid numbers pushing back the coalition and so they decided to gamble. The 500 elephants they had would charge straight into the Antigonid lines and it almost succeeded in breaking through but the coalition infantry was badly mauled in the process and they fled which caused chaos as the elephants went berserk and attacked both friend and foe but the mahouts (elephant riders) would either kill the elephants or regain control.
By nightfall, it was clear that the Antigonids won the battle of Ipsus as although they suffered moderate losses, the coalition's army was all but annihilated though it depended on which army as Cassander's army was destroyed, Lysimachus's army was badly crippled, and Seleucus's army was mauled.
The coalition would decide to leave Anatolia and defend their lands. Antigonos and Demetrius would split up with Antigonos going to the Levant and defeat Ptolemy and the remnants of the Seleucid army while Demetrius would move west and crush Cassander and Lysimachus.
300 to 295 BC: Lysimachus and his army of 20 thousand attempted to cross the Hellespont to Thrace but the Antigonid navy blocked his way and though his navy fought bravely they were unable to break through and so Lysimachus was stranded caught in between a sea and a massive army. The ensuing Battle of the Hellespont would see Lysimachus and his whole army being killed in a massive last stand however that wasn't the end of the Lysimachids as his son Agathocles was in the city of Lysimachia with the remaining troops of 10 thousand .
Antigonos with his army of 40 thousand would first move against Ptolemy and easily defeated the garrisons he put in place the Southern Levant by defeating them piecemeal instead of having them unite into a bigger force. Then he would move north to the Euphrates and march into Mesopotamia and Babylonia meeting light resistance from Seleucus who was making a new army in Persia. Antigonos would occupy Mesopotamia and Babylonia and put in several strong garrisons.
Demetrius would sail to Greece with his army of 50 thousand and finish off Cassander who could barely muster 15 thousand troops. In the ensuing battle, the exiled Epirote prince and Demetrius's brother in law, Pyrrhus would distinguish himself as a brave and competent leader who led the decisive right flank cavalry charge. Cassander would be captured and executed for killing Alexander's son and wife.
Then Demetrius would send Pyrrhus with an army of 10 thousand to retake the throne of Epirus as Demetrius moved into Thrace with 30 thousand troops. After sweeping through southern and central Thrace, he would move north to the last city, Seuthopolis and encountered Agathocles with an army of 25 thousand troops who were a mix of Macedonians, Greeks, and Thracians. Demetrius would bribe the Thracian contingent led by Seuthes III by offering him an independent Thracian state which he agrees to. In the battle, the Thracians would leave the field, leaving Agathocles very exposed and resulted in a crushing victory for Demetrius who lived up to his promise to Seuthes III by giving a state between the Haemus Mountains and the Danube though not the coast with the Greek colonies.
295 BC to 290 BC: Demetrius would sail back to Antigonia, Syria while leaving his son Antigonos II Gonatas to rule over the west. Demetrius and his father, Antigonos I would celebrate a triumph that lasted for a week and after it, Demetrius would march east and defeat the remaining Seleucid opposition,
In the east, Seleucus would launch raids to gain loot and to be a general nuisance. When Demetrius arrived with army of 70 thousand troops, the raids stopped and Seleucus adopted a defensive mentality mustering a force of 65 thousand troops mostly Persians with some Greek and Macedonians along with Bactrians, Sogdians, and a few Scythians and moving to a place of his choosing and also destroying towns and cities to deprive Demetrius of supplies.
Demetrius would occupy Susa and move towards Persepolis where Seleucus was waiting with an advanced guard of 10 thousand troops at the Persian Gates to halt the Antigonid army but Demetrius was able break through using catapults and other forms of artillery.
Seleucus would retreat to the north as Demetrius occupied Persepolis and then moved to chase Seleucus to Ecbatana. At the city, Seleucus stood with his entire army and the battle to determine the question of rulership of the East would begin.
Demetrius would advance, using his artillery to suppress the Seleucid archers. The fighting was fierce with casualties piling up on both sides but the Antigonids were able create a gap and flood through it causing parts of the Seleucid army to be surrounded and annihilated. Seleucus along with the remnants of his army would escape to Ecbatana and tried to move out at night but Demetrius would intercept the Seleucid column and utterly destroy it with Seleucus dying in the combat.
His son Antiochus would escape with any of the remaining troops to Bactria though they were a non factor as Demetrius would spend the remaining 4 years to conquer the east and establish Antigonid control of the region through putting in loyal governors, garrisons, and being hands off to the cultural practices of non Greeks.
When Demetrius returned, he was hailed as a hero, a new Alexander though tragedy would strike as in 290 BC, the now ninety one year old Antigonos the One Eyed died in his sleep.
Demetrius would deify his father and proclaim him as Antigonos Nicator or Victor though his epithet Monophthalmus would be used commonly. It would be common to have statues of Antigonos and Zeus side by side and coins would frequently have the two.
290 to 270 BC; Demetrius would deal with the remaining states in Anatolia and the states in the Arabian Peninsula which often ended with the Antigonids gaining better trade agreements and trading posts. He would crown his son Antigonos II as co-ruler to help manage the vast empire.
In the west, Pyrrhus was preparing to embark on a campaign to aid the Greek city states in Italy against an encroaching Roman Republic. He asked Demetrius for troops and money which Demetrius was all too happy to give Pyrrhus as they were close friends and Demetrius was married to Pyrrhus's sister.
Pyrrhus would land in Tarentum and defeat several Roman armies though each battle were very hard fought and costly in terms of men and money but this was mitigated by money sent from the Antigonids as well as Italic troops who were very angry with the Romans.
Meanwhile in Greece and Macedon, reports were coming in a massive horde of Celts moving south and weren't willing to be peaceful about it.
Antigonos Gonatas would muster whatever troops he had which was a respectable 20 thousand but the reports claimed that as many as 150 thousand Celts were marching. Gonatas would get envoys from the Dardanians, Thracians, and Illyrians for help which he accepted and now had 40 thousand but still to openly oppose the Celts in open battle was not possible.
By the time the horde arrived, Antigonos would avoid it and attack smaller parts of it until they either went away or were weakened enough to be defeated in open battle. The Celts would loot and plunder towns and cities if they were undefended which a lot were.
Many would leave though a 35 thousand troop contingent would stay behind and loot which Gonatas would ambush and annihilate it.
Next year, most of the Celtic army would come in again and led by Brennus, they marched straight through Macedon towards the Temple of Delphi. The other Celts would move east which Gonatas dealt with through diplomacy by giving them land in Central Anatolia which would help pacify the very divided territory.
Brennus and his army of 100 thousand would defeat the Greeks at Thermopylae and loot the temple but the Greeks regrouped and the Macedonians were blocking the way north which left them surrounded, out of supply, and in very hostile lands.
Brennus and most of the Celtic army was killed and despite their hostility towards them, their resistance had earned them the grudging respect of the Greeks.
Those Celts who settled in Central Anatolia would be loyal to the Antigonids for the most part.
In Italy, Pyrrhus and his army of Epirotes, Greeks, Macedonians, and Italians were defeating Roman armies in hard fought battles and after the battle of Neapolis, Pyrrhus would besiege the city of Rome.
The siege would last for 5 years and Pyrrhus would storm the city, razing it to the ground. The new Epirote Empire would let the Romans exist though in a small strip of land that spanned from Rome to the port of Ostia while the Italians who fought for Pyrrhus would be allowed to practice their own beliefs if they paid their taxes, joined the army for campaigns, and didn't rebel.
Pyrrhus would then get an offer from the Greek city states in Sicily to cast the Carthaginians back to Libya (that is what the Greeks called North Africa) which Pyrrhus was too happy to accept.
From 275 to 271, Pyrrhus with his army of 45 thousand troops would launch a lighting strike against the Carthaginians as they were still recovering from the wild adventures of the Syracusan King, Agathocles.
The war would be a stalemate with the Epirote army besieging the port fortress of Lilybaeum but they didn't have the navy to contest the seas with the formidable and huge Carthaginian navy.
In the year 270 BC, Demetrius the Besieger would die at the age of 67 and Antigonos II Gonatas would succeed him and crown his son, Demetrius II Syricus as co-ruler.
I do have more but I feel like this is enough.