In reality, just as Westmoreland's troop buildup had crushed the Vietcong, the politicians decided to withdraw, leaving the field open to a conventional invasion from the North. In Iraq, as in Vietnam, I'd argue that the insurgents were never in a position to to win, and the main threat was one of invasion. The difference in Iraq was that there's no way Iran could have invaded, without us responding, given the stranglehold on Gulf oil resources it would have given the Iranians.
Petraeus also decided it would be a good idea to have the troops get out of their armored vehicles more for the old hearts and minds crap. Result? More of our guys getting their legs and other parts blown off. In spite of the obvious stupidity of this move, the policy was continued for quite a while.
When Petraeus saw that Afghanistan was starting to deteriorate, he turned it over to some other guy and came back to the US as the conquering hero.
He’s a self-promoting jerk.