One might assume that this term is a 20th century one. After all, warfare in that century was often characterized by guerrilla combat. Guerrilla warfare is the choice of the oppressed seeking to throw out foreign oppressors, and the 20th century has seen lots of colonial wars. The term, however, is much older.
It dates to the Napoleonic campaign in Spain (1808-1811). The earliest English usage cite is by the Duke of Wellington in 1809 in his Dispatches 1799-1818:
I have recommended to the Junta to set...the Guerrillas to work towards Madrid.
In Spanish the word is a diminutive of guerra or war; so guerrilla is literally little war.
(Source: Oxford English Dictionary, 2nd Edition)
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