Manning yes. But before you get to Assange, there's a huge list of folks in our 'security' establishment that needed to be punished for incompetence and/or malfeasance.
Letting 3.5 million folks have access to this stuff was beyond stupid. It's like removing the compartmentalization of information at our nuke labs and letting the secrets walk out the door. It's so stupid to do this that it suggests intent.
This low level stuff is mostly embarrassing, but it's likely the same ideas were implemented for the higher classifications, that is they were made available to a much wider audience than before. If so, we have much larger fish to find and fry than Mr Assange.
Mr Assange could have been the type to sell this to Moscow (though I'm betting they already had it). But he did the one thing that is likely to get the problem solved. He published it where everybody could see it.
By definition, SECRET material is not compartmentalized within the American audience, although we do have restrictions on release for some of it to foreign allies ("SECRET NOFORN" etc.). To get any classified information, you should have an appropriate clearance and a need to know. With SECRET, need to know is more on the honor system. It is not deemed important enough to require strict compartmentalization, and the overhead that comes with it. When we need strictly enforced compartmentalization, as you talk about above, it would be classified TOP SECRET and put under "Sensitive Compartmented Information" (TS/SCI). Then someone has to confirm you have a legitimate business need to know the information and you get "read on" to the information. There are other various categories of compartmentalization too.
For the record, none of this is restricted knowledge. I'm going from past experience, but you can find all this in public sources on American government security practices.