Ooops!
According to Reuters, he never uttered those words.
But “Burke did say something resembling the quote in his “Thoughts on the Cause of the Present Discontents” (1770): “When bad men combine, the good must associate; else they will fall, one by one, an unpitied sacrifice in a contemptible struggle.”

‘OK, and where’s the difference?
The meaning’s the same…’

Not exactly!
Burke was speaking about the fate of individual people while the quote attributed to him is about evil itself.
According to Burke, the good people must associate in order to protect their livelihoods and their way of life while the mis-attributed quote pretends that there are circumstances in which evil might prevail.

‘I still don’t understand you! Good people loosing their cause doesn’t mean that evil has prevailed?!?’

No!
Good people might loose from time to time. Being good doesn’t mean those people are perfect.
People make mistakes. Some of which can’t be undone.

Evil things do happen. From time to time. Either through sheer bad luck or through good people making horrible mistakes. But evil cannot prevail. Not on the longer run!

For two reasons.

Small enough mistakes can be overcome. Either individually or collectively.
Serious enough mistakes will kill you. Individually and collectively.
This was the first reason.

If evil hadn’t been that bad, we wouldn’t have called it that way.
If evil would have led to survival, we would have called it ‘good’.

Good people doing nothing doesn’t mean that evil will triumph.
Good people doing nothing only means, as Burke had said, that those good people will fail. One by one.
Bad people having it their way doesn’t mean much. Historically speaking.
After they had finished vanquishing the good, the bad had always started to fight among themselves. It’s in their nature to do that.

That’s how each evil eventually dies out…

Until the next one appears?
Indeed! Weeds will always spring up. Specially if the soil is rich.
That’s what hoes are for! If only people knew how to make good use of them…