1580s, from L.L. retaliare "pay back in kind," from re- "back" + L. talio "exaction of payment in kind," from talis "suchlike." Originally used both in good and evil senses.

Pro- meaning intended? But how can you intend to retaliate something that was not really done ot given? Can you speak of retaliation at all?
How does pro- go with re-?