Imagine that Iran had a new government which wanted to get along with the outside world, and no longer constituted a nuclear threat either to its Sunni neighbors, Europe, Israel or the US. Given Iranian history, there is little doubt that such an Iranian government would look to the outside world for help. As the majority of Iraqis are Shiites, and as Iran is overwhelmingly Shiite, Iran’s government will be made up of Shiites irrespective of who rules that country; the Iraqis will still have to look to Iran as a big (though loathed) brother.
But if that new Iranian government were to be be allied to the West, what difference would it make whether Iraq had whatever it needed to defend itself? The Iraqis could then breathe a sigh of relief that both countries would be in the Western orbit and could look to the US for help in time of need.
So in helping to „liberate“ Iran, we would be „liberating“ Iraq as well: there would be no reason to worry further about Iranian influence in Iraq. And we could comfortably sell the Iraqis whatever they needed to defend themselves — without worrying about Iran’s influence there.