Alexander the Great died at age 86 not 33 and did not die of a fever. Because of this, he unified most the known world including Rome, Gaul, Hispania, and Carthage. He solidified the political structures and undertook transportation constructions to cement the Hellenistic Empire that spanned from Atlantic Ocean to the Pacific Ocean. The fusion of Greek and Persian cultures was more successful than that in OTL. His success resulted in almost two millenniums of an imperial dynastic system (similar to that in China) in a unified Europe, North Africa and Southwest Asia.
The unified political and cultural system strengthened the Empire to resist threats and rebellions. Economy was also more active because trading was protected within the Hellenistic empire. However, the unipolar structure also diminished regional diversity and damped cultural development in most places, although some such as Rome or Carthage flourished. Many cities also grew substantially in size during a span of 300 years under Hellenistic rule.
Conquests of the West
When Alexander the Great had conquered some of Asia, he controlled all he dared to dream of ruling, and would have been content to let his empire stay the way it was. However, in 307 BCE, a tribe of barbarians from Europe to the north raided and burned several of the northernmost cities and were barely defeated before reaching Pella, Alexander's birthplace and the Macedonian capital. Accordingly, in 306 BCE an army under Alexander's command ventured into the unknown forests of Europe. Alexander's army was unable to make much progress tracking down elusive barbarians and Alexander was forced to return to Macedonia, defeated.
Following the advice of Medius, the former king of Babylon, Alexander proceeded to spend lavishly to build cities in northern Macedon, thus "civilizing" southern Europe. His efforts were mostly successful, because, although barbarian raids still occurred, they could not get close enough to attack central Macedonian cities. For the next 5 years, Alexander rested from continuous warfare and took the time to improve the defenses of his empire and eliminate the barbarian threat.
Alexander the Great's Macedonian empire finally began to expand once more in 300 BCE, when the growing Roman power attacked Western Macedonia and attempted to usurp his empire. In response, Alexander led an army 85,000 strong in an amphibious assault on Rome itself. The army was successful, but Alexander was wounded and taken out of the action for a while. By the time he was fit for combat again, his army had conquered northern Italy and the Roman exiles had put together an army of 65,000 in Sicily to face Alexander's force of 60,000. Alexander achieved a crushing victory and managed to destroy the Roman Empire after defeating the exiles in Sicily and the Alps.
With the destruction of the Roman Empire, the Carthaginian Empire took back Sicily (except Syracuse) and captured more of Spain. Although Western Egypt had surrendered to Alexander in exchange for being united with Eastern Egypt, the Carthaginian Empire fought bitterly back against Alexander. In 294 BCE, Alexander attempted an amphibious assault on Carthage, to quickly cut off the head of the Carthaginian empire, but his troops were beaten back and forced to sail away. Instead of sailing back to Italy, Alexander simply landed at the western border of his Egyptian possessions and began a ground invasion. This took significantly longer, but by 291 BCE, most of the Carthaginian empire was in his hands with the capture of all of North Africa. Carthage (and Rome) became great economic sites and experienced significant growth in the second century BCE.
In 290 BCE, Alexander once again sailed his army into another country. This time he attacked Spain, where the last of the Carthaginian Empire was still existing. By now, the Carthaginian empire had little in the way of conventional troops, and only a moderate sized navy, but with assistance from Celt-Iberian tribes they maintained a guerrilla warfare throughout the Pyrenees Mountain Range. This guerrilla warfare continued until 288 BCE when Macedonian troops from northern Italy defeated the European tribes in the area to the northeast of the mountains, thus cutting off the supply lines for the guerrillas. In 286 BCE, the Carthaginian Empire was defeated and annexed.
At this point, Alexander the Great had achieved his goal of controlling every port into the Mediterranean Sea, so he at first resisted attacking outside his borders, and concentrated instead of fortifying and securing his new empire. In 284 BCE barbarian raids close to Pella, provoked Alexander, leading a campaign against northern Europe. In order to counter the guerrilla tactics of the indigenous residents, Alexander built a network of forts throughout Gaul that quickly grew into small towns, thus urbanizing the area. The Gallic tribes saw the wonders of Alexander's empire and many tribes peacefully joined and provided soldiers to assist Macedon. Finally, in 279 BCE all of Gaul to the Rhine and southeastern Europe to the Dniester River was under Macedonian control.
By the time Alexander had conquered all of this territory, he was 77 years old and in no shape to continue campaigning. Alexander returned to his capital at Pella and allowed his troops, under Hephaestion, to continue across the Rhine and into Briton without him. Although his troops were victorious, their performance in battle suffered without their general, and Alexander decided against future conquests. In 277 BCE Alexander moved to Ravena, Italy and started a wine shop called A'la Carte. It was very successful until the city was attacked by pirates midway through 277 BCE. Alexander traveled back to Macedonia, and took up residence there. In 276 BCE, at the age of 80, Alexander the Great turned his empire over to his son, Alexander IV, then 39 years old. Alexander lived for six more years before dying in 270 BCE, as the creator of the greatest empire the world had ever seen.
By installing his son on the throne, Alexander the Great ensured that his empire would endure and even expand. Alexander IV "the Fine" conquered the Brythonic tribes in present day England and Wales, and expanded the Hellenistic Empire until it controlled all of OTL modern Germany and parts of the Czech Republic and Poland. After conquering the Picts, Scots and Gaels, the Hellenistic Empire gained both Scotland and Ireland. Future heirs, most notably Karanos I who conquered Kush and the Slavic tribes, expanded on this empire until, by CE 29, it spanned from the Western border of OTL Russia, south to Nubia, west to the Atlantic, and east to the Pacific Ocean.
In 128 CE, and republican form of government was established with a Legislature ruling alongside the Monarch, much like that of Eighteenth Century OTL United Kingdom. This lasted until the eventual collapse of the Hellenistic Empire.
For the next 200 years, the empire held steady, but the appearance of the Mongols in Northwest Asia created severe strains on the empire. Although the Mongols were defeated by CE 242 and rooted out of OTL India, the empire had been badly shaken and parts of the Eastern Asian territory had been pillaged. The Mongols would return repeatedly, but never for more than 300 years an extremely severe threat to the empire's existence.
Division of the Empire
By CE 245, the dynasty that Alexander set up was declining. A succession of weak rulers combined with the drain of resources caused by the Mongol attacks plunged the empire into economic and political turmoil. In 257, the empire that Alexander the Great spent his entire life building was divided into many separate states: the Empire of Persia in the Middle East, the Republic of Babylon, and various states in Europe and North Africa. The largest three are the Roman Republic, Carthaginian Republic, and Kingdom of Egypt.
The Roman Dynasty
In CE 261, after several attempts by Rome, they were finally successful in establishing Roman Dominance over the Italian Peninsula. After this, the Romans united Italy, Northern Spain and the western parts of the Balkan Peninsula into the Roman Republic, all by 266 CE. Carthage had done the same, and had united North Africa and Southern Spain by 268 CE. In 279, Rome warred with Carthage and took Sicily with very heavy losses, which after after this a white peace was agreed on.
After a long period of relative peace between the two superpowers, Carthage conquered the Kingdom of Egypt in the 290s, and Rome conquered most of Gaul, and southern Germania by 295 CE, but the Brythonic tribes in OTL England had consolidated a large fleet from the remnants of the Hellenistic Empire, and was able to resist Roman attacks for several centuries more.
Germanic invasion and rise of the Byzantine Dynasty
While the Romans were occupied with attacking Britain they did not realise that the Germanic tribes were mustering in Northeast Germania to regain their lands lost to Rome. Under the Visigothic leader Theodoric the Bloody-Handed they poured across the Elbe and took the Rhine garrisons by surprise, the Roman tenth legion made a heroic last stand at Castra Vetera but were annihilated, other legions were easily overcome, by 298 CE, they had re-conquered Germany.
The Roman general Flavius Scipio was notified while preparing an amphibious assault on Londinium. He took a fleet away from Britain which landed at the mouth of the Rhine, already Theodoric was besieging towns across northern Gaul and had crushed the governor of Lutetia at Alexanderomagus in 300. Scipio attacked Theodoric from behind and chased him to Brittany where a Germanic navy picked the barbarians up and sailed offshore, preparing to land at Burdigala.
Scipio was presented with a choice whether to pursue Theodoric by sea or to march over land to fortify Burdigala. He chose to go by sea as it was quicker, this decision altered the course of the war. While sailing past Portus Namnetum Theodoric's admiral, Freawine appeared from a small bay and ambushed the Romans, Scipio won the day but was killed in action and without a good leader the Roman navy was easily destroyed by Theodoric himself later that year. The Germanic and Brythonic tribes were pressing hard on Gaul and Roman legions were destroyed one by one until Northern Roman Gaul was no more.
Theodoric landed at Burdigala in 303 and easily captured the town, Freawine pushed south and crossed the Pyrenees into Roman Spain (to the south was Carthaginian Spain), he annexed it as part of the New Germanic Empire and it joined Gaul as a barbarian kingdom under the leadership of Theodoric. The Romans abandoned their conquest of Britain and planned to recapture Gaul and North Spain under the new leadership of Alexander Byzantinus, a direct descendant of Alexander the Great.
In 304 after some serious planning Byzantinus landed at Palermo in Sicily, he gathered reinforcements from Italy and sent a spy to Freawine who was ruling North Spain on behalf of Theodoric. The spy turned Freawine against Theodoric and civil war broke out in the New Germanic Empire, hence Byzantinus got the name Propositumus or 'scheming'. The civil war dragged on until 306 when Byzantinus invaded Spain from the south with permission from Carthage, who themselves were planning an attack on Germania.
The weakened Germanic forces were defeated at Gades, Corduba, Emerita Alexander, and Castra Legionis at which Freawine was captured and executed. Due to his popularity with the people Byzantinus was proclaimed king by his troops in 307 against the present ruler Romulus Marcianus. A Carthaginian General, Nacifel Caso, sailed to the Balearic Islands where he defeated Marcianus' forces in a decisive battle and then annexed Sardinia and Corsica from Germania where he caught the Germanic king and killed him, and gained back the islands for Carthage. After this, Byzantinus was crowned king of the Roman Empire, renamed the Byzantine Empire in 310. He reinstated the Macedonians as the ruling class and began the Byzantine Dynasty of Macedonians.
In 310, Byzantinus invaded Gaul where Theodoric was ruling with an iron fist. He was beaten at Arausio but undeterred continued and won two victories at Lugdunum and Lutetia where Theodoric was captured and imprisoned. By 314, Byzantinus had nearly all of Gaul but let the Germanic tribes keep Germania. He celebrated a great triumph at Pella in which he pardoned Theodoric to show he was merciful. He ruled until 359 and his dynasty continued long after that.
Carthage to the south consolidated its land in North Africa: Southern Spain, North African Coast from OTL Western Sahara to Somalia, Corsica, Sardinia, Cyprus and parts of Arabia (Israel and Saudi Arabia). Carthage would continue to exist until shortly after the sack of Rome. Although struggles with Saharan tribes in the third century had weakened Carthage, this only led the strengthening empire led it to ally with Rome, and eventually conquer much of the Sahara, especially around the Niger River.
In 390 CE, a huge invasion of Mongols defeated the Kingdom of India and they shortly after annexed it into their empire. They went on to capture Tibet, Southeast Asia, and the northern parts of China, even though China armies fought viciously to the death. Fortunately, the Mongols took huge losses, but they expansion rate rivaled that of the Hellenistic Empire's speed of capture of the Persian Empire. This had all happened by 400 CE.
The Persian Empire once again met a speedy end in 407 CE after Mongols progressively captured the Arabian Peninsula, East Persia in two years and West Persia and Judea in another two years, although the Persians put up a good fight and their leaders retreated to Macedonia and merged the remnants into the Byzantine Empire. The Mongolian Empire, the Carthaginian Empire and the European Empire were now equals.
After major preparations, the Mongols directly invaded Macedonia in 410 CE from their strongholds in Asia Minor. Their generals advanced over 300 km into Macedonia and captured Byzantium, and once threatened the capital city of Pella, but war legions soon drove them back 100 km.
A massive attack by the Carthaginian Empire in revenge of the capture of Judea and Cyprus numbering 100,000 drove the Mongols 800 km back out of Arabia, but the once very successful offensive soon settled into a stalemate, even though the Mongols were busy fighting on three fronts (African, Chinese, European).
After making peace with the Chinese in 421 CE, the Mongols surged into Arabia and retook most of it back from the Carthaginian Empire, although they were not able to take back the Sinai Peninsula. In response, the Carthaginian Empire's navy landed in Judea, and established a foothold that grew every year until its pillaging by Mongols in 433 CE.
Meanwhile, the Byzantine Empire's greatest general of this century, Honorius Qembe, prepared for a huge assault on Asia Minor and Crete which had recently been captured by the Mongols. In 426 CE, Qembe landed on Crete and surprise attacked the Mongols with few losses. However, a trap set by the Mongols however caught Qembe by surprise, and a huge skirmish began. The Mongol cavalry were able to surround his legion before he could make a move and he narrowly escaped the battle. Eventually his soldiers were defeated, although they took heavy losses with them. Qembe sailed to Athens to regroup and fortify the city.
By 427 CE, Mongols were capturing Carthaginian Egypt, with was putting heavy resistance, and were preparing an attack on Byzantium. Qembe's legion was gaining strength and exceeded 80,000 men at this point. When the Mongols had landed in Greece, Qembe was ordered to counterattack in Crete while another general fought the Mongols. Once Qembe's troops landed, he soon found that Mongols had established several forts on the island. He led an attack on the three located on the west side of the island, although re-inforcements of cavalry archers soon arrived and attempted to use the same tactic that they had used to defeat Qembe before. This time, Qembe countered the cavalry with his own cavalry armed with spears, which resulted in heavy losses for both sides. Qembe eventually succeeded in taking the forts, but by 428 he was having trouble.