I imagine they'r referring to the video of a helicopter shooting someone in Iraq who turned out to be holding a camera rather than a rocket launcher, which Wikileaks released under the title 'Collateral murder.'

If you ask me the real war crime is that the administration we had at the time went to war in Iraq in the first place, based on an apparent mixture of personal animus between Presidents Bush & Hussein and an alarmingly thin piece of social engineering by an Iraqi criminal/defector known as 'Curveball' who wanted to (at best) destabilize the Saddam Hussein regime or (at worst, but quite understandably) evade prison in Iraq. This entirely optional conflict led to hundreds of thousands of deaths, or by some counts >1 million, while (IMHO) significantly prolonging the war in Afghanistan by turning that theater into a sideshow.

http://en.wikipedia.org/wiki/Curveball_(informant)

http://www2.gwu.edu/~nsarchiv/NSAEBB/NSAEBB234/index.htm

I don't think Manning's information releases revealed very much about the war in Iraq other than to shed light on this particular helicopter incident which I think falls rather short of being a war crime, although I do think it was worth bringing to public attention.

On the other hand, I see no public benefit whatsoever in teh release of all the State Department cables, and as far as I can make out Manning took them and Assange released them just because they were classified by default. When their full contents became available because of carelessness with the crypto key outcome was some diplomatic red faces but there sure wasn't any big reveal of criminal activity.