In 390 B.C., Gaul sacked Rome, butchering its elder statesmen, pillaging its homes, and lighting the city on fire. Rome would not be sacked again for 800 more years, until Alaric led his band of Visigoths down through Italy. Not even Hannibal with his elephants and his military genius breached its walls.
Julius Caesar eventually brought Gaul to heel, conquering a divided Gaul in the Gallic Wars. Over the course of Caesar's 8-year wars, one million Gauls died (about 20% of the population), another million Gauls the Romans enslaved, and 800 cities lay smoldering in ruins. The Gauls fought back; yet the Romans were too strong and too organized, so won. Nevertheless, from this conquest, Roman Gaul emerged, an advanced, stable, comfortable society.
By the time the Suevi, the Vandals, the Alani, and the Burgundians invaded Gaul in 406 A.D., Gaul was a nation transformed. Although Gaul was filled with healthy young men, unlike when Caesar invaded, it put up no resistance to its Barbarian invaders. This time, a small band of invaders massacred large masses of young men who did not resist, raped its women, and burned and pillaged its towns and cities. Within three years, the invaders had become masters of the unresisting Gaul.
What happened between the time when the Gauls traded conquests with the mightiest civilization on the Earth, versus when they rolled over and died for a paltry band of rapscallions?
The most obvious answer is they got comfortable.