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How about Donald Rumsfield?

Rumsfeld’s comment that: “…this insurgency is going to be defeated not by the coalition – it’s going to be defeated by the Iraqi security forces, and that is going to happen as the Iraq people begin to believe that they’ve got a future in that country”.

While we are waiting for that to happen ...

“Lt. Col. Frederick P. Wellman, who works with the task force overseeing the training of Iraqi security forces, said the insurgency doesn’t seem to be running out of new recruits, a dynamic fuelled by tribal members seeking revenge for relatives killed in fighting. ‘We can’t kill them all,’ Wellman said. ‘When I kill one I create three’.”

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Your quotes don't show that senior military officials think the war in Iraq is unwinnable. They show the importance of having US forces in Iraq training Iraqi forces.

The top brass of today's military are either Vietnam veterans or those who joined the military just after Vietnam. They remember how bad our post-Vietnam military was, and how much better it is now. If they thought that the war in Iraq was unwinnable and was slowly destroying our military, you would see stories quoting unnamed Pentagon sources about the need for a US exit from Iraq all over CNN/Fox/MSNBC, ABC/NBC/CBS news, and all the print media.