The quotation most frequently attributed to Edmund Burke is not something Burke said though it sounds like Burke. The correct quotation from the Thoughts on the Cause of the Present Discontents, and found on the home page of this website, is different. Professor David Bromwich of Yale University explains why the difference is important:
‘When bad men combine, the good must associate; else they will fall, one by one, an unpitied sacrifice in a contemptible struggle.’ This great sentence is the germ of the most famous quotation wrongly ascribed to Burke: ‘The only thing necessary for the triumph of evil is for good men to do nothing.’ The sentiment extractable from the corrupt version is pompous, canting, and demonstrably false, for evil is not a disembodied thing; it has its origin in acts by specifiable agents. Nor is it true that ‘the only thing necessary’ for the triumph of evil is the inaction of good men. There must also be an extended occasion of public fears for bad men to play upon, and there must be a catalysing event. The bad men themselves must be unusually excited, active, conscious of each other’s presence, aware of the inlets for increasing power, and unimpeded by the indifferent mass of people. Burke’s sentence is careful to say flatly what the triumph-of-evil apothegm leaves mysterious. Bad men do combine but, as the word combine suggests, their alliance may be impersonal and almost mechanical, a reflex of ambition and appetite, or the product of a theory. By contrast, association, through constant intercourse with other persons, leaves room for correction and improvement in the corps and a concern for the public good. Yet it matters that the defence of principle, when its cost is high, should achieve a public notability, so that interim defeats may prepare for an eventual triumph. The sacrifice of a party is important because it is so visible, compared to the obscure sacrifice of an unconnected person. Consistency of opinion, which is both a cause and a consequence of regular association, makes all the difference between a contemptible and worthy struggle.
The intellectual Life of Edmund Burke: From the Sublime and Beautiful to American Independence, David Bromwich, pp. 175-176