When the leaks happens, point to a few cables and claim that they were fakes.
By insisting that these cables will cause "immeasurable harm" and put "countless lives at risk", the USG is granting credibility to these cables and hurting themselves. Instead, fight information with disinformation.
Anyway: that would be my advice.
So if you claim that you're doing that, it's an obvious bluff.
Or it's entirely possible that they really believe (or know) that the information will cause harm and risk, and are doing their best to prevent it. My understanding is that the State Department is usually staffed with people who want to do the right thing. In fact, most of the US government is.
Seriously, based on how often people cry foul whenever it appears that some government is trying to spin/manipulate information, it's probably a better idea that they didn't try that.
And the fact that you advise that points out to something. Your advice is probably the correct course of action for any body out to maximize it's self interest. But people generally don't like that in their governments. Hell, they don't even like when their governments are cold, calculating and cynical.
The fact that they did not state so simply adds credibility to wikileaks case, that these are fanciful allegations without any evidence whatever.
In one instance, it appeared the US Ambassador to Russia/Georgia had been invited to a local tribesmen's son's wedding and goes on to detail every bit of information about his encounters, very confidential chats and even forecast what will happen in power struggles because of his conversations with various top ranking guests.
In the cable he even verbally outlines the structure of the house, how many exists there are, location etc.. as if to mark a military strike on the location should there ever need be.
All of this led me to beleive this ambassoder is royally fucked. The fact that Washington has suggested embassy counselors will be called back/removed from their post is probably very true and otherwise, these kind of information leak will put them in great danger.
I'm not sure how WikiLeaks determines what information, such as names, should be censored while other names are fully visible. This is quite disturbing to me.