Final answer:
This quote reflects the historical view of Gauls as brave warriors but psychologically fragile in adversity. The exact source is unspecified but aligns with classical texts discussing Roman-Gaulish conflicts.
Step-by-step explanation:
The quotation "Just as the spirit of the Gauls is prompt to engage in war with great alacrity. to the same degree their mind appears fragile and poorly suited to sustain calamity..." can be traced back to historical texts that discuss the characteristics of the Gaulish people about warfare and their psychological endurance in the face of disaster.
While the exact source of this quote could not be pinpointed, it reflects the historical perspective of the Gauls as brave and eager warriors who may not withstand adversity well. This view is illuminated through various depictions of Gaulish warriors, both victorious and defeated, throughout classical historical sources and texts discussing the Roman encounters and conflicts with the Gauls.