Why is irregular warfare called a 'little war'?

Because the Spanish word guerrilla is the diminutive of guerra, 'war': literally, a 'little war'. The diminutive evokes the gap in numbers, scale and scope between small irregular bands and the formal, professional army of a state. The term became popular during the Peninsular War of the early 19th century: after their regular armies were defeated, the Spanish and Portuguese people rose against Napoleon's troops and defeated a highly superior army, combining guerrilla strategy with a scorched-earth policy and a people's war.

The word was new; the tactics were not. Guerrilla warfare means small groups of irregular fighters using ambushes, sabotage, raids and hit-and-run attacks against regular forces. In China, Sun Tzu proposed guerrilla tactics in The Art of War in the 6th century BC. In the 3rd century BC, the Roman general Quintus Fabius Maximus, nicknamed Cunctator, 'the delayer', evaded battle, tried to wear down Hannibal's army and ambushed small detachments, a method known as the Fabian strategy. The Han dynasty general Peng Yue, who attacked enemy convoys and supplies, is often regarded in China as the inventor of guerrilla warfare.

English got the word from the British general Arthur Wellesley, who adopted it from Spanish usage in 1809, at first for the individual fighters. In correct Spanish, though, a fighter is a guerrillero, or a guerrillera, and in most languages guerrilla still names a style of warfare. That style rests on one calculation: avoid head-on confrontation with a better-armed or larger enemy, and fight limited skirmishes to exhaust it and force it to withdraw. Organized guerrilla groups often depend on support from the local population or from sympathetic foreign backers.