How much worse things in Iraq would be now if the US forces had simply stopped after wiping out Saddam and just left. Keep as much of the bureaucracy (the part that actually does the work of running the country) intact, same with the bulk of the armed forces - again, the ones that actually do the work. Give a grant of 10 billion to the government, to be held by Switzerland or someone until there is a recognized election.

There would have probably been a lot of sectarian violence.

I think that the Sunnis, Shias and Kurds would have withdrawn to their own areas and formed separate enclaves, if not formal countries.

But without the US forces to focus on, I think things might have sorted themselves out before now.

I'm sure there are other opinions.

Of course, that didn't happen, but there's no law here against indulging in alternate histories.