A variation of the Lanchester equations is applied to exploration of the force ratios required to win in guerrilla-counterguerrilla warfare. It is shown that an attacking guerrilla force can, by using tactics that compensate for its weaknesses, defeat a force of defending regulars that has over-all superiority in numbers and weapons. The defenders can win by appropriate selection of weapons, countertactics, and group sizes for individual engagements. In general, however, the high over-all ratios of defending regulars to attacking guerillas that have characterized recent warfare of this kind in which the defense has been successful are extremely difficult to reduce.