Ah, the argument from ignorance. It's my 'favorite' logical fallacy, inasmuch as it seems to be the go-to fallacy of the dumbest people making the dumbest arguments. It's the Nuclear Option of bad logic; when all else fails and every single shred of evidence is against someone's argument, you can rest assured that this bad boy is about to be whipped out.
An argument from ignorance is, quite simply, "There is no evidence for X, therefore not X" or "There is no evidence against X, therefore X." This fallacy is frequently paired with a sub-fallacy regarding evidence which holds that only direct observation can prove or disprove something. Combined, they make an extremely pernicious fallacy in politics because so much of what gets debated is not directly observable. We don't get to sit in on many White House meetings, do we? And we don't spend a lot of time eyewitnessing events in Iraq, do we?
I like to call this sub-fallacy the "red handed" fallacy, because it is often argued that unless you can provide first-hand evidence for something (even if it is well-supported by circumstantial evidence) then you must be wrong. If you can't produce a signed affidavit from George Bush stating that he didn't consider the potential for sectarian strife (or if you weren't in the room while they debated it), then the pre-war planning is unimpeachable. If you can't provide documented proof that pre-war intelligence was distorted, fabricated, or cherry-picked – something on the order of a video of Cheney saying "Let's lie about the intelligence!" – then they were telling the truth. If you didn't see a murder being committed, then you can't say that the accused is guilty (forget fingerprints, DNA, eyewitness testimony, or any other evidence). These arguments all rest on the assumption that if we can't directly observe X then we can't prove it…from which the leap to X being necessarily false is easy. For idiots.
Am I being unfair? Perhaps this is the kind of nonsense one only hears from Rush Limbaugh call-in guests and it's unfair of me to depict it as a larger problem. Well…
For instance, what specifically does it mean to say that the Iraq war has worsened the "terrorism threat"? Presumably, the NIE's authors would admit that this is speculation rather than a statement of fact, since the facts suggest otherwise. Before the Iraq war, the United States suffered a series of terrorist attacks: the bombing and destruction of two American embassies in East Africa in 1998, the terrorist attack on the USS Cole in 2000, and the attacks of Sept. 11, 2001. Since the Iraq war started, there have not been any successful terrorist attacks against the United States. That doesn't mean the threat has diminished because of the Iraq war, but it does place the burden of proof on those who argue that it has increased.
Ladies and gentlemen, that actually got published in one of the nation's widest-circulating and most well-regarded newspapers. Read it again. Honest to f'n Christ, that got printed. Not to be outdone (by himself, apparently) Kagan squatted over the national discourse, pants around his ankles, and pinched off this sludgy brown loaf of wisdom a few weeks ago: "The 'Surge' is Succeeding." His logic, in case you can't bring yourself to read the whole thing, is that we have no evidence that it isn't working, so it is working. Reading an extended 'argument' from Kagan is like swimming the backstroke through a 100m trench of broken glass and saltwater.
This is probably the most appropriately-named fallacy, since you can be quite certain that "ignorant" is a fitting adjective for anyone who makes this sort of argument.