In the year 2007, after the horrible war on Iran a young man travels back to the year 2005 - He wants to convince Bush not to attack the Iran... Let us assume that Bush believes that the time-traveler told him the truth. In the best of all possible worlds he will then immediately calls C. Rice so that she stops all intended attacks. But then it is unclear why the time-traveler is still standing in Bush's office - at least there is no more motivation to travel back from 2007 to 2005. Assuming that there still must be some motivation, Bush has also to believe that the war on Iran will happen - whether or not he now calls Rice. But then it seems rather doubtful why Bush should belief what the time traveler says and why should act accordingly.
Note that this reasoning assumes that there is only one time-line, such that events either occur or do not occur. (Probably a reasonable assumption, but it's worth noting nonetheless.)
I think in the above scenario, the astute time-traveler would also contact his past self to notify him that he must (in future) travel back in the past to warn Bush in order to prevent the war. In such a case, the war might never have actually happened at all. Perhaps the only reason Traveler goes back in time is because his future self (whom he trusts) had earlier told him to! Such 'loops' are troublesome, however, since it isn't clear how they could start in the first place. (Where is the ultimate cause? It's all so circular.)
[I'm reminded of an old post from DTWW which summarizes a Heinlein story involving a fascinatingly devious causal loop.]
But what if we allow for multiple timelines? Perhaps in one timeline there really is a war, but when Traveler goes back in time the universe 'branches' in some sense, so that he successfully brings about a new world where the war does not occur. Is that possible? Does it properly count as time travel, if he's not really traveling back into his own past, but rather creating a whole new one?
The attraction of this view is that it frees time-travelers from the strict demands of their own timeline's past. I want to say that once events in a particular timeline are determined, each moment is forever fixed and cannot be changed. If something occurs, you cannot make it not occur. So it's impossible to actually change the past through time travel. All you can do is fulfill your 'destiny', carrying out those actions that your future self has (in a sense) already done. That is, all you can do is make your causal contribution to bringing about those past events that already occurred.
So it's impossible for Traveler to prevent a war that has already happened. Either it never occurred in the first place (as in the loop described earlier, where he only ever went back in time because his future self told him to), or he didn't really change his past, but rather created some altogether new one.
Does that sound plausible? I'm fairly short of sleep, so I can't be entirely sure that I'm making sense. And the topic does lend itself to insensibility. For a moment I wondered whether branching timelines could explain (i.e. originate) causal loops. But on second thought, that really doesn't make sense at all.