Go here for a fuller explanation of the real significance behind Bush’s recent shake-ups in the top ranks of the Pentagon. A sample:
Petraeus was also a supporter of Cheney’s proposal for striking Iranian Revolutionary Guards Corps targets in Iran, going so far as to hint in an interview with Fox News last September that he had passed on to the White House his desire to do something about alleged Iranian assistance to Shi’ites that would require US forces beyond his control.
At that point, Fallon was in a position to deter any effort to go around DoD and military opposition to such a strike because he controlled all military access to the region as a whole. But Fallon’s forced resignation in March and the subsequent promotion of Petraeus to become Centcom chief later this year gives Cheney a possible option to ignore the position of his opponents in Washington once more in the final months of the administration.
For still more on why Bush and Cheney find Petraeus such a useful tool for locking the next president into their idiot war, see this.