"Who breaks a butterfly upon a wheel?" is a quotation – sometimes misquoted with "on" in place of "upon" – from Alexander Pope's "Epistle to Dr Arbuthnot" of January 1735. The line has entered common use and has become associated with more recent figures.
It alludes to "breaking on the wheel", a form of torture in which victims had their long bones broken by an iron bar while tied to a cartwheel. To do the same to a butterfly is quite some 'overkill'. Hence, it can be taken as referring to putting massive effort into achieving something minor or unimportant.
The line "Who breaks a butterfly upon a wheel?" forms line 308 of the "Epistle to Dr Arbuthnot" in which Alexander Pope responded to his physician's word of caution about making satirical attacks on powerful people by sending him a selection of such attacks.
Let Sporus tremble –"What? that thing of silk,
Sporus, that mere white curd of ass's milk?
Satire or sense, alas! can Sporus feel? Who breaks a butterfly upon a wheel?"
Yet let me flap this bug with gilded wings,
This painted child of dirt that stinks and stings;
Whose buzz the witty and the fair annoys,
Yet wit ne'er tastes, and beauty ne'r enjoys,