'The Plan' was Afghanistan, Iraq, Syria, and Iran (with more to come later as necessary).
The Yanks started making fairly serious noises concerning Syria and Iran soon after George Bush announced "Mission Accomplished" in Iraq.
However some Iraqis begged to differ, which put the plan in doubt. Then the Afghans started making a nuisance of themselves, and the plan went out of the window altogether.
Syria was probably always going to come before Iran, but only because they knew Iran was would be such a hard nut to crack. Basically the weakest first, and as experience and confidence grew, the stronger ones would be dealt with. And there really isn't any comparison between Iraq and Iran in that respect.
……….. well imo anyway.