When assessing the acts of a famous conqueror like Alexander, it's tempting to look at him as if he existed in a vaccuum. We can say, look, there he is in Macedon; here's a whole rich fertile landmass that he's already on, that he can easily overrun, that later became the core of several large empires. He's got the finest army in the world and there's no major European power to stand in his way. Why would he choose to turn east instead, and cross the sea and a bunch of mountains and deserts and take on the largest empire in existence?

But if we did this, we would be ignoring the world in which Alexander lived, and the legacy of his father, Philip II.

First of all, the Greek world was centred around the Mediterranean, more than any continental landmass. All Greek traders, mercenaries, settlers and armies that had ever gone anywhere had always done so along the coasts and nearby inland areas of this great sea, which allowed for quick and easy travel. Any power that made its presence known on the sea was familiar to the Greeks, but any that dwelled inland was essentially irrelevant to them. At the time of Alexander, the Greeks had settlements all over the Mediterranean and the Black Sea, but they had only a vague notion of the peoples of inland Europe. As far as they were concerned, the lands north of those inhabited by the Thracians and Illyrians of the Balkans were cold, dark, inhospitable places with very little to offer. Expansionist Greeks were much more likely to turn their gaze west or east.

Indeed, in terms of returns on investment, attacking north-northwest into Europe would have been a rather senseless proposition. The wealthiest parts of the world were not the rainswept lands of the Getai in Romania or the little plains of local wranglers like the upstart city of Rome. They were, in the west, Sicily and Carthage, and in the east, Ionia (on the west coast of modern Turkey) and Egypt. These were prizes worth having.

In terms of political justifications, there was certainly reason enough to turn west and attack Carthage. The Carthaginians had been openly hostile to the Greeks in Sicily for a century and a half, and had periodically subjected large swathes of the island to their rule. However, as a target for a Macedonian conqueror, they were less attractive for three reasons. First, the fiercely independent and powerful Greek cities of Southern Italy and Sicily would not gladly submit to Macedonian overlordship, even if it meant freedom from Carthage. Second, these very cities had only recently (in 339/8 BC) defeated the Carthaginians on Sicily and pushed them back to a couple of tiny footholds, regaining freedom and democratic constitutions for themselves. Third, Carthage was a naval power, and to challenge them would require substantial investment in a large, state-of-the-art fleet that would have to operate far from the Macedonian homeland. This matched neither the style nor the scale of Alexander's ambitions.

Even if we consider Alexander's campaign as a mere personal imperialistic project, then, the choice to go east seems perfectly obvious. In 20 years of campaigning, Philip II had already conquered all of Macedon's immediate neighbours - the Molossians and Epirotes to the west, the Illyrians to the north, Thracians to the east, and the mainland Greeks to the south. The entire territory of modern Greece, Albania and Bulgaria was already subject to him. Alexander had to reassert his rule over these areas after he ascended the throne, but once this was done, there was really only one target worth going after. If Alexander went west or north, he would face an uncertain future for little gain. If he went east, into Asia Minor (modern Western Turkey), he would:

• secure the trade routes through the Hellespont (which his father had failed to do despite long and costly campaigns);

• secure the Aegean Sea, on which the Macedonian heartland bordered, as a Macedonian-controlled lake;

• secure the wealth of the Greek cities of Asia Minor;

• ideally capture the Persian satrapal city of Sardis and seize the wealth stored there;

• create possibilities for further expansion eastward into the Persian Empire.

It's worth stressing that the wealth of the Persian Empire was already legendary at this time, and that all other prizes paled in comparison to the vast quantities of loot to be had if there could be even some local victories against the Persians. For centuries, all the surplus money of the largest empire in the world had been flowing east. In the course of a few years of campaigning, Alexander "liberated" in excess of 200,000 talents of gold from Persian treasuries - many times the compound revenue of the Athenian Empire for the whole 70-odd years of its existence.

But the best part of it was that it wasn't just his own crazy dream. People were begging him to do it.

The hatred between Greeks and Persians went back to the initial conquest of Ionia in 546 BC; it was brought to a fever pitch by the Persian Wars of the early 5th century BC. Throughout the 150 years before Alexander, the Greeks repeatedly tried to liberate the Greek cities of Asia Minor from Persian rule, but the Persians very effectively managed to assert their control over the region. Athenian imperial power was broken with Persian funding in 404 BC; the Spartans' attempt to free the area of Persian influence was decisively crushed in 394 BC, and the treaty imposed upon the Greeks in 386 BC (called the King's Peace) stipulated that the Greeks of Asia Minor were subject to Persia.

In this period, we see the rise of the movement called panhellenism. This was a political ideology of sorts, which aimed to bring constant Greek in-fighting to a halt and to persuade the city-states to join forces against the common enemy. In their view, the eastern barbarians were the ancestral, fated enemies of the Greeks, who had been their inveterate enemies since the days of the Trojan War. To fight them was the only just war. The Spartans already liked to portray their imperialist efforts in the 390s BC as a panhellenic crusade; later authors like Isokrates insisted that Athens should lead the way. But their pleas went unheeded. The Greeks kept fighting among themselves, and whenever any one state became too powerful, the Persians immediately stepped in to reassert the terms of the King's Peace and keep the Greeks divided. By the 360s, it was clear that only a powerful autocratic ruler could carry out the panhellenic agenda. Some proposed Iason of Pherai, but his tyranny proved too unstable. Not so the monarchy of Philip II.

A magnificent speech from Isokrates survives, in which the Athenian orator tried to stave off the inevitable conquest and subjection of the Greek states to Macedon by persuading Philip to turn against Persia. Philip was smart enough to realise, though, that he could not safely cross to Asia without first subduing the Greeks at home. He achieved the conquest of mainland Greece at the battle of Chaironeia in 338 BC, uniting the subjected city-states into the League of Corinth.

The express purpose of this League was to carry out the panhellenic crusade and liberate the Greeks of Asia Minor.

This was the situation Alexander inherited when he came to power. Once his rule was firmly established, it was all but predestined that he should go east and invade the Persian Empire. If he didn't, the League of Corinth would rightly revolt from him, and he would get bogged down in endless wars trying to hold on to all the little bits of the kingdom that Philip had built up. But if he did, he would unite the Greeks in purpose; he would legitimise his rule by fighting the common enemy; he would give his subjects opportunities for glory and enrichment; and he would emulate all the great heroes of the Greek past, all of whom had turned east to fight the barbarians there.

The panhellenists had hoped only to liberate the Greeks of Asia, and possibly to create a buffer zone of mercenary settlements from Sinope to Kilikia, cutting off Asia Minor from the Persian Empire. But Alexander's thirst for glory far exceeded the task they had in mind for him.