A rhetorical question referring to an excessive amount of force that has been applied to achieve something minor, unimportant, or insignificant. The line is a quotation from Alexander Pope's poem "Epistle to Dr Arbuthnot." To "break upon a wheel" refers to a mode of torture, in which a victim has their bones broken while strapped to a large wheel. The government's use of drone strikes and artillery bombing on the town to wipe out a tiny faction of rebels is totally unjustifiable—who breaks a butterfly upon a wheel?