Final answer:
The guerrillas were successful due to their guerrilla warfare tactics and lack of French military support. The exact number of casualties suffered by the French during the Peninsular War varies, but it was not as high as 300,000.
Step-by-step explanation:
The guerrillas were successful during the Peninsular War because they employed guerrilla warfare tactics, which included hit-and-run raids, ambushes, and small-scale engagements in difficult terrain. This type of warfare made it challenging for the French forces to combat them. The guerrillas operated independently of the French military and did not have their support.
While the Peninsular War was a costly conflict for Napoleon, the exact number of casualties suffered by the French Empire varies depending on the source. However, the number you provided, 300,000, is an exaggeration. The French did endure significant losses but it is important to note that they did not lose 300,000 troops during the Peninsular War.