Surely the largest part of the "blame" has to lie with the US authorities for putting such sensitive cables somewhere where so many people (Bradley Manning being only one of them) could access them?
That was done in response to the fallout from 9/11, where it was demonstrated that there was very little inter-agency cooperation, especially in sharing information about terrorism/terrorists.
It was a calculated decision to greatly increase information sharing between government agencies (including the military) to allow for better cooperation.
Well... it seems they did not calculate accurately the permission controls to prevent people like Manning making and distributing a copy of all those files.
I think it's more appropriate to blame the second law of thermodynamics and the internet. It's increasingly, if not impossibly, hard to keep information isolated.
My suggestion to anyone with secrets that valuable is to assume they get leaked and have a plan for that.
I'm curious if Wikileaks saw this coming, but I suspect they were naive enough to believe they could maintain control.
