The dog attacked the mailman unprovoked, without any reason or warning.

Something that has been provoked means that it has been caused or led on by something that has been done or said. Unprovoked means not caused by something that has been said or done.

It depends what is meant by "unprovoked". An emu always has a reason for whatever it does, and what may seem unprovoked to humans is not necessarily so to emus. An emu will only attack if it believes there is a threat to its chicks or itself, so stories of "unprovoked attacks" on cars or people are probably inflated. For whatever reason, the emu in question was feeling threatened at the time.

Pre-emptive strike