At its height, the Persian Empire encompassed most of Mesopotamia, Asia Minor, the shores of the Black Sea, some of Central Asia (what would one day become Afghanistan), parts of the Caucasus mountains, Egypt, Thrace, and Macedon. For nearly two centuries, Persia had mastered the “cradle of civilization”—until a Macedone upstart named Alexander crushed the world's greatest power in less than four years.

Just as notable as its ignominious collapse was its improbable start. According to several accounts (all of them suspect), it all began when a young Cyrus II couldn’t get along with his grandfather Astyages. To be fair, the old king did try to have Cyrus killed at birth … so when Cyrus proclaimed himself king of Persia in 546 BCE, his grandfather’s lands were first on the list of conquest. By roughly 540 BCE he overran Lydia and a year later marched in triumph to capture Babylon. He now ruled an empire that reached from the borders of Egypt to the shores of the Black Sea, encompassing all of ancient Mesopotamia.

By all accounts, Cyrus II—now known as "Cyrus the Great," or the “King of Kings”—was, for his time, an relatively enlightened ruler. His Persian Empire was the first in history to govern a slew of distinct ethnic groups on the basis of equitable responsibilities and rights for each, so long as his subjects paid their taxes and kept the peace. Cyrus established a system of local nobles called "satraps" to administer each province autonomously, and pledged not to interfere in the local customs, religions, and economies of the conquered peoples. He built fortresses along the eastern border to limit the depredations of barbarians from the steppes, such as the Scythians, who may (or may not) have been the cause of his untimely death in 530 BCE.

Cyrus was succeeded by his son Cambyses II, who promptly murdered his own brother Bardiya to ensure his rule would not be challenged. As was the style at the time, he followed fratricide with an invasion. Cambyses marched his armies to Egypt in 525 BCE, winning victories at Pelusium and Memphis, but his attacks on neighboring Carthage and Nubia did not fare as well. Nonetheless, Egypt and its wealth was a nice addition to the empire. In the midst of all this campaigning, Cambyses received word of a revolt against him led by his brother, Bardiya—the same one he had previously murdered. On the way home to make sure his brother's death took this time, Cambyses himself died under mysterious circumstances.

Coincidentally, a distant relative of Cambyses named Darius—one of the Persian generals with access to his king around the time of his death—claimed that Cambyses took his own life out of despair. Darius marched his troops to Media and killed Bardiya, who he labeled an imposter. Without a clear line of succession to dissuade him, Darius claimed the throne, leading several provinces to immediately revolt at his presumption. After 19 battles in a single year, Darius had put paid to most of these uprisings.

Tranquility returned to the empire. Darius was in firm enough control by 521 BCE that he reorganized the administration, made Aramaic the official language of the sprawling empire, and created a uniform monetary system based around the “daric,” because naming a coin after one's self was one of the perks of (presumed) usurpation. Under Darius, the Persian Empire also standardized weights and measures, instituted a program of road construction, such as the rebuilding and completion of the 1677-mile “Royal Road” from Susa to Sardis, and began lots of public works in the cities of Susa, Babylon, Memphis, Pasargadae and new Persepolis.

By 516 BCE, he considered his rule stable enough to invade the distant Indus Valley, which he conquered within the year. After appointing the Greek Scylax to serve as his satrap from the city of Gandhara, Darius decided to reorganize the empire. He divided it into 20 provinces, each under a satrap he appointed (usually one of his relatives), and each paying a fixed rate of tribute. To prevent the satraps from building a power base to threaten rebellion, Darius appointed a separate military commander in each satrapy, answerable only to him. Imperial spies (known as “King’s ears”) kept tabs on both satrap and military commander, reporting back to Darius on a regular basis.

Having returned from India victorious, Darius (now known as “the Great”) turned his attention to the Scythians. The horse barbarians refused to engage in a pitched battle, but their constant withdrawals cost the Scythians their best pasture lands, scattered their herds, and lost them several allies. Still, the Persian infantry were suffering from fatigue and privation themselves after a month of marching into the wilderness. Darius, concerned that the fruitless campaign would only cost him more men, and convinced that Scythian fortunes had been damaged enough, halted his army on the banks of the Oarus. According to Herodotus he built “eight great forts, each some distance from each other." After declaring his stalemate a victory, he departed to seek far less mobile foes in Europe.

The expedition began with Darius crossing the Hellespont and getting involved in fractious Greek politics. This led to the invasion of Thrace, followed by the capture of several city-states in the northern Aegean. Macedon submitted voluntarily to Persia, becoming a vassal kingdom. Darius left his general Megabyzus to finish off Thrace while the king returned to relax at Sardis. But it wasn’t long until a number of Greek cities in Ionia revolted and, supported by Athens and Eretria, an Ionian force captured and burnt Sardis in 498 BCE—to the presumed irritation of Darius.

With the “Ionian Revolt” finally and utterly crushed, Darius sent his son-in-law to reconquer Thrace and Macedon in 492 BCE, then dispatched an expedition to Greece to force Eretria and Athens to submit to his will. After island hopping across the Aegean, overwhelming Naxos on the way, the Persians besieged, captured and burnt Eretria in 490 BCE. They then marched south along the coast of Attica, looking to do the same to Athens, but were thoroughly defeated by 30,000 allied Greek soldiers at Marathon. Darius immediately began preparations for another invasion, this time planning to personally take command, but died three years into the effort.

His successors—starting with his son Xerxes I—were left to deal with the upstart Greeks. And, despite ruling the greatest empire yet known to the world, they managed to bungle it.

Xerxes first quelled a revolt in Egypt, but unlike his predecessors, Xerxes dealt harshly with the rebellious province by removing the local leaders and imposing direct Persian control on the citizens. He did the same to the Babylonians when they revolted in 482 BCE. Finally, Xerxes led a great army into northern Greece, supported by a powerful Persian navy. The city-states in his path fell to the invaders fairly easily, and despite a heroic stand of Spartans and Boeotians at Thermopylae, the Greeks were unable to stop Xerxes’s army from marching to Athens and sacking the most important city-state in Greece.

However, the Athenians had evacuated their city before the Persians arrived, and their navy remained a potent force. Xerxes learned just how potent at the battle of Salamis in 480 BCE, when a Greek fleet of some 370 triremes soundly defeated 800 Persian galleys, destroying perhaps 300 Persian vessels at a cost of 40 Greek ships. This defeat delayed the planned Persian offensive further into Greece for a year, giving the Greeks time to strengthen their defenses against the invaders. Xerxes was forced to return to Persia, leaving his general Mardonius in command, and the Greeks promptly won several important naval and land battles against the new leader. With Mardonius's death in the battle of Plataea, the campaign was over and the surviving Persians withdrew from Greece in disorder.

Xerxes never mounted another invasion of Greece, though this was more due to his assassination than a lack of interest. In 465 BCE he fell victim to a plot engineered by the commander of his own royal bodyguard, who was in turn was killed by Xerxes's son Artaxerxes.

Ruling from 465 BCE to 404 BCE, the three Persian kings who followed Xerxes I—Artaxerxes I, Xerxes II, and Darius II—were weak and uninspiring. At the end of the 5th Century BCE the Persians regained some power in the Aegean, successfully playing the Greeks against one-another during the long Peloponnesian War between Athens and Sparta. However, an Egyptian revolt in 405 BCE wrested the wayward province out of Persian control for more than 50 years.

Darius II was succeeded by Artaxerxes II, who ruled for 45 years. During his long reign Artaxerxes II fought a war against Sparta, once again over the Greek colonies in Asia Minor. Persia allied with the Athenians (who were recovering from their disastrous defeat in the Peloponnesian War) and Sparta was forced to come to terms.

Despite these occasional successes, Persian weakness and disorganization grew as the 4th Century BCE progressed. In 373 BCE a group of satraps revolted. They were put down, but other revolts followed, and with growing frequency. The position of king was increasingly unstable. Artaxerxes III came to the throne as a result of treachery in 359 BCE, and in an attempt to secure his position he promptly murdered as many of his relatives as he could find. In 338 BCE Artaxerxes III was poisoned at the orders of the eunuch Bagoas, who placed Artaxerxes's youngest son Arses in power. Arses promptly tried to kill Bagoas, but his effort failed and he himself was killed. Bagoas then elevated Darius III to the throne.

A former satrap of Armenia, Darius III was only distantly related to the late king(s)—but nearly everybody else with a better claim was dead. It is difficult to tell if he was an effective leader, for the Persian Empire had been in decline for well over a century by the time he assumed the throne. Its many component parts were in near-constant revolt against the increasingly inept central government. Palace intrigue further crippled the monarchy, and leaders who wished to survive spent as much time watching their backs as they did looking out for the interests of the empire. Any leader who took power under those circumstances would be in trouble.

However bad things were at home, they paled into insignificance with the troubles headed Darius’s way from across the Hellespont. In 336 BCE a young Macedone king named Alexander, later labelled “the Great,” sought to topple the tottering Persian Empire. Darius repeatedly met him in battle, often with far superior numbers, and Alexander simply destroyed his armies one after another. The Persian capital Persepolis fell to Alexander’s armies in 330 BCE, and Darius was murdered the same year. The last Achaemenid “King of Kings” had fallen.