Tuesday, February 14, 2012

Project: Impossible — Julius Caesar at the Siege of Alesia, 52 BCE

Gaius Julius Caesar

My 26th book will be Project: Impossible, an exploration of how people achieved goals any reasonable person would have thought impossible. Here’s a summary of some of the cases covered in the book.

The Rise of Vercengetorix

Until the rise of Vercengetorix, Gaius Julius Caesar had been able to fight the tribes of Gaul one at a time. But in 52 BCE, they united under a single leader: Vercingetorix, chieftain of the Arverni tribe.

Caesar’s military and political situation at the time was deteriorating badly. Caesar’s political enemies, known as the boni, threatened him in Rome, and this new uprising compromised his plans for Gaul.

Vercingetorix conducted one of the first known uses of a scorched earth policy, destroying crops to keep them from falling into the hands of the Romans. He also dopted a hit and run strategy. In addition, he raised an army many times larger than the Romans who opposed him — by some counts, as large as 500,000.

Caesar, distracted with events in Rome, was in the settled Roman province of Cisalpine Gaul when Vercingetorix opened his campaign, but quickly crossed the Alps with eight understrength legions to find the scorched earth policy beginning to bite. Although Vercingetorix had burned twenty settlements, he had spared one, the fortress town of Avaricum, thought to be impregnable. In a 27-day siege, plagued by poor supplies and surrounded by hostile Gauls, Caesar took the town — and the food.

Vercingetorix, in response, captured a food convoy bound for Caesar. Still determined to avoid a decisive battle until the odds favored him, he retreated his cavalry into the fortress town of Alesia.

Vercingetorix had every reason to believe that his situation was still advantageous. His forces outnumbered Caesar’s. He had the advantage of high ground. Most importantly, the defending forces inside Alesia were only a small part of the Gallic forces. Soon, Caesar would not only have to contend with the forces inside Alesia, but also the remainder of the army of united Gaul — a relief army of between 125,000 and 250,000. Caesar would shortly find himself trapped in a doughnut, with enemies both inside and outside.

Caesar’s response was to launch one of the most ambitious and astounding feats in the history of military engineering.

The Impossible Project

First, Caesar’s men built a circumvallation, an eleven-mile long fortification of earth piled thirteen feet high, enclosing the entire town. Behind the earthen rampart his soldiers dug two ditches, each about fifteen feet wide. If that wasn’t enough, Caesar’s men built 23 fortlets, one every 80 feet, along the entire route — and did it in only three weeks!

Of course, Caesar also had the Gallic relief forces to worry about, so now he had to do it all over again. The Roman forces built a contravallation, an external set of defenses similar to the circumvallation, but this one extending for thirteen miles!

This immense engineering feat took thirty days, slowed by the need for Caesar’s men to collect supplies to feed the army. But it was all done before the huge relief army arrived.

A reconstruction of Caesar's fortifications around Alesia

The Battle of Alesia

After skirmishing and small battles, the main attack began at midnight, with Vercingetorix’s men crossing the treacherous fortifications Caesar’s soldiers had built. Caesar’s legates Marc Antony and Gaius Trebonius (later one of Caesar’s assassins) were able to repulse the attacks from both sides.

Meanwhile, the leaders of the relief army scouted Caesar’s fortifications and found a weak spot, a Roman camp to the northwest that had not been included in the contravallation because of the hilly terrain. Two legions (around 8,000 soldiers) occupied the camp, and the Gauls sent an attacking force of nearly 60,000 against it, starting with diversionary attacks before the major assault began. Vercingetorix, seeing some of the preparations, launched an attack on the inner lines.

Vercingetorix Surrenders to Caesar
Caesar himself waded into the thick of the battle, and the Romans carried the day. The next day, Vercingetorix surrendered. His men were sold into slavery.

Managing the Impossible Project: Maximizing Resource Quality

The performance of the Roman legionary is legendary, and it’s not surprising that 30,000 Romans could defeat a force arguably ten times as large. But even a cursory reading of Roman military history will make it abundantly clear that not all Roman generals enjoyed equal success.

Of course, Caesar’s military and engineering genius had a lot to do with his success, but it’s the superior performance of his legions, even by already high Roman standards, that is the key to understanding Alesia. The staggering magnitude of the earth-moving alone is a testament to backbreaking, unromantic work. It’s one thing to convince soldiers to fight; it’s another thing to convince them to dig.

If there is a mismatch between what you want people to do and what they actually are doing, you can either modify the process or modify the people. Modifying the process may mean improving the tools and equipment, or it may involve changing methodologies. Modifying the people can involve motivation, or changing the rewards and punishments for performance.

When people are well trained, motivated, and led effectively, they can achieve results that would otherwise be impossible.


  1. War is a fascinating subject. Despite the dubious morality of using violence to achieve personal or political aims. It remains that conflict has been used to do just that throughout recorded history.

    Your article is well done, a good read.