Why did he do it?

So now we've met Vercingertorix.

By the end of January he had formed a coalition of eight neighbouring tribal states and was amassing a militia army to equal Julius Caesar's fifty thousand men. At the time there were ten Roman legions in Gaul - the 2nd, 7th, 8th, 9th, 10th, 11th, 12th, 13th, 14th and 15th - the first five were Spanish, and the last five were Gallic. Gallic? That's right, half of Caesar's army was made of Gauls, at least twenty-three thousand of them. In fact, the only Italians in Caesar's army were his officers. So this leads us into the murky area of who was a Gaul back then. Well, there were the two Roman provinces for a start - Cisalpine Gaul - where the Gallic legions came from in now northern Italy. Then there was Transalpine Gaul - modern day Provence in France. Both of these had been part of the Roman Republic long before Julius Caesar was born. The parts he actually invaded were Celtic and Belgic Gaul. And until 52BC, Celtic Gaul - most of modern France - had been relatively trouble free and loyal to Rome - Caesar had no reason to believe things had changed. But they had. What Vercingetorix did in less than a month was unite half of the Celtic tribes (about 2-million people) under his military government. Caesar's success had been playing off the states against each other to gain their support, Vercingetorix turned that around.

How?

When the revolt began six legions were camped on the River Seine south of Paris. Another two were further south in the Loire Valley and two were in Belgium. But Caesar was in modern Piacenza hedging his bets there would be a civil war in Rome that year rather than trouble in the Celtic states. And he wasn't the only one. The Celtic Gauls thought Rome would consume itself that year. The Senate House had just been burnt down by a mob and the senate elections had been postponed. Those like Vercingetorix knew this was the only chance they would get to rid themselves of Rome. Against any other Roman General at the time, they would have been right. There was only one commander who would have braved the Alps in the middle of winter to get back to his legions - the trouble for the Celtic Gauls, it was Caesar...

Find out if Calvus crossed the Alps