During which military conflict was the term "guerrilla warfare" popularized?

The term, the diminutive form of "war" in Spanish, is usually translated as "little war", and the word guerrilla has been used to refer to the concept as early as the 18th century. It became popular during the early-19th century Peninsular War, when the Spanish people rose against the Napoleonic troops and fought against a highly superior army using the guerrilla strategy.