[94] From Egypt and Greece to Spain and Britain, the Roman world seemed united. And, it seemed the Roman Empire would last forever. But there was just one small flaw in the plan: people.

[95, busts mostly from Capitoline Museums; Palazzo Massimo alle Terme, National Museum of Rome; Vatican Museums, all in Rome] After centuries of relative stability, Rome began its fall — partly because of some bad emperors. Sure, some were good and some were mediocre, but many were downright evil. As they were invested with immense, almost god-like power, with egos as supersized as their empire, they left plenty of portrait busts. There were empire builders like Augustus, wise leaders like Hadrian, and crazy rulers like Caligula.

[96] The last great emperor of the Pax Romana, Marcus Aurelius, sits on his horse atop Capitoline Hill in Rome. The original (housed nearby out of the acidic modern air) is a rare surviving equestrian statue from antiquity and a great symbol of Rome at its peak.

[97] Reliefs — as if part of a PR campaign — show off the emperor ably performing the duties of state, keeping his empire humming: As the chief priest, or "pontifex maximus," he sacrifices a bull. He vanquishes the barbarians and parades through Rome on a chariot, trumpets proclaiming the glory of Rome…winged victory on his shoulder.

[98] But after Marcus Aurelius, Rome was ruled by an unfortunate string of ineffective, incompetent, and now-mostly-forgotten emperors. The son of Marcus, Commodus, was a cruel tyrant. He declared himself a god, dressed up like Hercules, and clubbed innocent subjects to death. By this time, the fall of Rome was inevitable. But its decline was gradual, stretching across two centuries, and even during its fall, Rome produced some wonderful art.

[121] Despite the stabilizing influence of Christianity, Rome's decline was inevitable. The fall of Rome had many causes: There was a string of terrible emperors. Especially near the end, emperors were routinely assassinated. The infrastructure crumbled, critical ports silted up. Subjugated people — the so-called "barbarians" — rose up. Rome's legions were sent backpedaling as the once-invincible empire gradually shrank. Even the walls of Rome itself were breached and the city was looted and sacked vandalized by a tribe actually called "the Vandals."

[122, Hadrian's Villa, AD 118–138, Tivoli, near Rome] Finally, in 476, the last emperor checked out, flicked out the lights, and plunged Europe into a political vacuum — ushering in centuries of relative darkness — poverty, chaos, and war.

[123, excavation at Largo Argentina, Rome] The once-great capital city of a million people, repeatedly ravaged by barbarians and plagues, eventually lost over 95% of its population. The place where Rome began — the Forum — was abandoned, later nicknamed the "cow field." Rome's grand structures crumbled, were built over, and eventually got buried under centuries of rubble, silt, and today's modern city.

[124] Yes, Rome fell. But its spirit lived on: in the Latin language. In laws and literature. In the Roman Catholic Church. And in its monumental art and architecture — grandeur that would inspire Europe for centuries to come.