On some level, Jeb and most Republicans know that the Iraq War wasn't a glowing success. Maybe Dick Cheney is still 100% convinced that it was the right choice and everything worked out perfectly. But most Republican presidential candidates aren't giving a full-throated defense of the decision. In fact, many are implicitly acknowledging that it was a bad decision when they say stuff like what Jeb said:
“I would have [authorised the invasion], and so would have Hillary Clinton, just to remind everybody,” Bush told Fox News television in an interview to be aired late on Monday. “And so would almost everybody that was confronted with the intelligence they got.”
You'll notice that many people are falling back on the "intelligence at the time" justification as to why they would have invade Iraq. Putting aside the problems with what the intelligence at the time actually said, even if the intelligence would have been perfect and told us definitively that Iraq did or didn't have nukes, this is bullshit, as Daniel Larison explains:
Even if administration claims had been right, there was no threat to the U.S. or anyone other country that warranted an invasion. The main problem with the war was not that the U.S. and its allies failed to prepare for the aftermath of regime change (though they did), but that they launched a “preventive” war on shoddy evidence for the explicit purpose of toppling another government by force.
The decision to invade was indefensible, and the war was entirely unjustified. It could not have been salvaged or made better by a more competent occupation, but then there is no reason to think that the previous administration or any American administration would be competent at establishing a new system of government in another country that it barely understands. The fact that Bush can’t begin to grasp that the original, irredeemable error was the invasion itself tells us all that we need to know about his appallingly bad foreign policy judgment. It gives everyone fair warning that he would make the same sort of disastrous blunder if presented with the opportunity. That alone proves him to be unfit for the presidency.
Jeb Bush, nor really any Republican or conservative pundit, hasn't grappled with the real reasons invading Iraq was a mistake. This is apparent in how they talk about Iran's nuclear policy. To them, any country with nuclear weapons is automatically a direct threat to the US. There's always a mushroom cloud around the corner. They haven't learned a single thing from the entire Cold War or the more recent Iraq War. They have no clue as to how countries with nukes or those pursuing nukes have acted. And they are of course blindly dedicated to supporting Israel.
Which is all to say that their entire worldview or philosophical underpinning when it comes to foreign policy is completely skewed by paranoia, much like their domestic philosophy. This isn't to say that anyone who isn't as fanatical as most of the GOP isn't going to be hawkish; see Barack Obama, Hillary Clinton, and many other Democrats. But with Democrats you at least have a chance at them making the right decision. If Republicans win the presidency in 2016, we will be left crossing our fingers and hoping that they just luck into not making horrible foreign policy decisions.
Update: Chris Christie says he wouldn't have invaded Iraq knowing what we all know now. But, of course he didn't really mean that because he says Bush made the right choice at the time. No wonder Christie's campaign can't get going. Aside from being a complete asshole and probably corrupt, he tries to play to whatever moderate conservatives he thinks still exist.