There's a difference between winning the war and winning the peace.
In Iraq, we won the war hands-down. We went in to effect regime change and bring Saddam to justice. We did exactly that. We toppled Saddam's regime, established a new one in its place, and then captured and executed the man himself.
However, we lost the peace badly. We failed to anticipate the rebels, the influx of al-Qaeda, the sectarian civil war, the rise of ISIS, etc.
Couldn't you say the same for the Vietnam War? After the Tet Offensive, North Vietnam was reeling from it's failure to achieve its military objectives (it was an absolute success politically).
The US got North Vietnam to agree to a peace treaty and then left South Vietnam. Vietnamization failed and once the North Vietnamese realized the US wasn't going to help the South anymore, they just took the whole country.
In Iraq, we won the war hands-down. We went in to effect regime change and bring Saddam to justice. We did exactly that. We toppled Saddam's regime, established a new one in its place, and then captured and executed the man himself.
However, we lost the peace badly. We failed to anticipate the rebels, the influx of al-Qaeda, the sectarian civil war, the rise of ISIS, etc.