Alexander the Great is often said to be the greatest conqueror and best military leader to ever walk the face of the earth. He conquered many territories, creating one of the largest empires in history. He was a very kind and generous man, and he also had a good sense of humor. All of his troops respected and liked him, and almost all of his captured territories served him willingly, because he was kind to them, and protected them from any attacks. Alexander was not a pushover, however. He massacred any opposition of his rule (usually with little loss of life on his side), and he brutally silenced anyone who refused to obey him. He was not a leader like Darius of Persia, who watched his army battle from miles away, or from the very back, Alexander rode right up in front with his army, and stayed with his soldiers even in the heaviest fighting. Sadly his power crumbled after his death, because he left no successor to his empire.
On July 20, 365 BC, Olympius, the wife of King Philip the Second of Macedonia, gave birth to a son and named him Alexander. This boy would eventually become the greatest military leader in history. On the day of his birth, it is said that the temple of Artemis burned down, thus indicating a good omen for Alexanders future greatness. The true date of his birth is unknown, but the most widely accepted one is July 20, because he centered many important events on or very near that date later in life.
As a child, Alexander often became discouraged when he heard that his father had conquered another territory. He worried that nothing would be left for him. He spent much of his childhood around his fathers army, so by the age of thirteen, he had become very mature. I think the story of how Alexander captured his prize horse, Bucephalus, at the age of thirteen demonstrates his maturity and bravery. Philip had bought an incredibly beautiful horse, but he was so fierce, no one could touch him. Just when the men were ready to give up, Alexander arrived and bet thirteen talents (a lot of money for a thirteen-year-old) that he could tame the horse. He calmly approached the horse and realized that it was afraid of its own shadow. By riding Bucephalus into the sun and slowly turning him around, he was able to ride the horse. This horse became his best friend and when he died, Alexander named a city after him.
Alexanders mother, Olympias, was a princess of Epirus (a captured city) who after seeing her city fall, fell in love with Philip, and later married him. She was said to be brilliant, hot-tempered and extraordinarily beautiful. She taught Alexander that the great Achilles was his ancestor, and that he should strive to be like him. She had Alexander read the Illiad, the story of the feats of Achilles, and to his dying day he always kept a copy of this story with him. Achilles became Alexanders role model, and his ambition was to be as brave, kind, and mighty as Achilles. Alexanders Father, Philip, was king of Macedonia and ruler of many territories. He was said to have once been handsome until the effects of war took their toll, scarring him horribly. He was an incredible military commander, very ambitious, and a good speaker and leader. Alexander was said to have his mothers good looks and cunning, and his fathers ambition and military prowess.  
Alexander parents always wanted the best for their son. They hired some of the finest tutors around, including the brilliant Aristotle, who began to teach Alexander around the age of thirteen. Aristotle taught Alexander the ways of the Greeks, which he incorporated into his life thereafter. Alexander found interest in philosophy, ethics, other countries, politics, plants animals, military, and a wide range of other subjects. After Aristotle, his parents employed Alexanders uncle, Leonidas. Alexander hated Leonidas because he was very strict, so this tutor did not last very long. Alexanders final tutor was a man named Lysimachus. He taught the young prince the cultural aspects of the world around him, and gave him an appreciation for fine arts such as music, poetry and drama. He also taught Alexander to play the lyre. By his late teens, Alexander had become very intelligent and well rounded young man.
Alexander rose to power quickly, and at an early age. His father gave him many large responsibilities, even at the young age of sixteen. When King Philip went away he always left Alexander in charge, giving the young man a good chance to prove himself to his father. One time while Philip was away, one of the captured colonies revolted. Alexander quickly took control of the situation by assembling and marching troops to that area. Also by the age of sixteen, he had founded his first colony, Alexandropolis. 
Alexanders rise to full power came in June of 336 BC when his father was assassinated at the theater. Alexander was crowned king of Macedonia and all of its territories at the age of twenty, taking the title Alexander the Great. His first task as king was to suppress the uprising in the large city of Thebes. He commanded his troops magnificently, destroying almost every building and taking almost 30,000 as slaves. This victory with almost no loss of life broke the spirit of rebellion in most of Greece, and many colonies and cities pledged their allegiance to Macedonia soon after. 
The ambitious young king then turned his thoughts to Persia. His father had planned to take Persia, but he never got around to it before his death. Alexander knew he could never be dominant in his area as long as Darius, king of Persia, was alive. In the spring of 336, Alexander crossed the Hellespont with a force of 35,000, planning to have an easy victory over the Persians. He met the Persian army on the Banks of the Granicus River, easily decimating their infantry with his cavalry. This victory basically opened up all of Asia Minor to Alexander.  
Darius, angry at his defeat on the banks of the Granicus, challenged Alexander to a battle on the banks of the Issus River (which he had already fortified.) He had also raised a larger army this time than his last encounter with the Macedonian king. He was expecting an easy victory, but Alexanders phalanx infantry strategy proved to be too powerful for the Persians, whose defenses buckled under Alexanders heavy infantry. Alexander captured Darius and his family, and just when they were expecting to die, he treated them like royalty in his court. This gallantry and kindness toward his archenemy greatly improved Alexanders standing in many peoples eyes. Although he treated the Persian king nicely, Alexander knew that they could not both rule Asia, so he warned Darius that next time he would not be treated so well.
Next Alexander tried to take Tyre, which proved very difficult for his armies to capture. The city was on an island, so for him to effectively besiege the city he had to build a causeway across the channel while under heavy arrow barrage from the Tyrians. This proved very difficult, and the Macedonians and their allies suffered many casualties, but in the end, Alexander took the city, slaying over 8,000 Tyrians and taking almost 30,000 in slaves. Gaza, another large city, also refused to serve Alexander. They suffered the same fate as Tyre, except with far fewer losses on Alexanders side.  
Next, Alexander went to Egypt. To his surprise, the Egyptians welcomed him as a savior, because Darius had treated them so horribly. They had heard tales of his power and his kindness, so they pledged service to him willingly. He founded a city on the strip of land between Lake Mareotis and the Mediterranean Sea, naming it Alexandria. This city later became a world center of knowledge and learning. 
Meanwhile, Darius had amassed an enormous army for his final showdown with Alexander. The Persians cleared and smoothed an enormous plain near Arbela, East of the Tigris River. The Persian cavalry outflanked Alexanders right and captured his camp, but with a charge that Alexander himself led, the Army was re-routed and retired to their camp, knowing they had been defeated. With his defeat, Darius fled to the mountains, only to be assassinated by one of his own noblemen. With his death, Alexander was crowned King of Persia, and thus, King of all Asia. 
The city of Babylon surrendered, and Alexander easily captured the Persian cities of Persepolis and Susa, which yielded him vast amounts of gold and silver. Every inhabitant of Persepolis was either killed or taken as a slave, and Alexander burned the entire city as revenge for the Persian burning of Athens in 480.
Alexander marched on, his only opposition being a few small groups of local tribesmen. His Empire had gotten very large, too large for him to control alone. He set up groups of governors that he knew he could trust in each of his conquered territories. These governors were almost all loyal to Alexander. The only exception was the Iranians who revolted against Alexander, who very quickly silenced them. After his conquest of Persia, Alexander went to the Caspian Sea, and then down to Afghanistan, Bactria, and Sogdiana, setting up governors and strongholds as he passed through. It took two years to pacify this region.
After strengthening his army with Iranians, Alexander turned his attention to the rich plains of India. He defeated an Indian Prince, Porus, and planned to march down the Ganges River, but his army mutinied because they thought he was pushing them to hard. Instead, he sailed down the Indus river, and marched across the desert of Gedrosia (in present day Pakistan) to take all of northern India. 
Alexander had now reached the height of his empire, which stretched from the Ionian Sea all the way to northern India. He decided to make Babylon his capital city and to make Asia and Europe one big country, combining the best of the east with the best of the west. He now had no time for conquering; all of his time was spent organizing and administrating his vast realm. He encouraged intermarriage, marrying Persian princess, the daughter of a Sogdian baron, and many other influential women as an example. His army consisted of men from all of his territories, and he had a general from each province. He set a universal currency throughout his land and encouraged trade and commerce. He also encouraged the spread of Greek ideas, laws, and customs into Asia. He was widely liked by the citizens of his realm for his kindness, fairness, and respect for people of lesser importance than he and was feared for his temper and military strength.
Alexander had vast plans, including governmental reorganization and an expedition to Arabia. Unfortunately, none of this ever happened. Alexander the Great died of malaria while in Babylon on June 13, 323 BC. His soldiers mourned the loss of their great leader, whom almost all loved and respected. They embalmed him and put him to rest in a gold sarcophagus, which was taken to Memphis, Egypt, and later taken to its final destination in Alexandria. Sadly the tomb did not remain untouched. Ptolemy IX needed money, so he melted down the great kings solid gold sarcophagus to make gold coins. People were so enraged that Ptolemy would do such a thing to a legend, he was executed soon after.
Along the span of his lifetime, Alexander conquered many regions in a short length of time. His great character and values were another thing that caused him to be remembered as a prominent figure in history. Some of his most notable accomplishments were the seven-month long siege of Tyre, and the final battle against the Persians, in which Alexanders forces were greatly outnumbered. One of his amazing, yet less remarked on accomplishments was simply making it through each battle alive.
As I said, living through all these battles was pretty amazing, considering the fact that Alexander always rode with the front line of troops. He suffered many minor and major injuries, ranging from a case of mild hypothermia while crossing the Cydnus River, to a bird dropping a stone on his head. In the battle against the Mallians, his troops thought their leader had been killed when an arrow pierced his lung. The troops were so enraged, they stormed through the city killing almost all its residents in a bloody massacre. While invading Samarkand, Alexanders leg bone was split, making it impossible for him to ride back to safety. The cavalry got the honor of carrying their precious leader back home, but soon, the infantry got jealous so Alexander decided to let each unit take turns sharing the honor.
Unfortunately, Alexander left no successor. His Generals became leaders of various areas and fought among themselves for control of the region, but no prominent leader emerged victorious. In 311 BC, all of his empire had split into individual states or monarchies.