Saddam regularly executes army officers. He does it as a sort of premptive measure. Sometimes the victims are really plotters, sometimes they're just bystanders. In either case it serves as an object lesson to everyone else.
There's a chilling video floating around of Saddam shortly after he took power. He's in an auditorium with most of the Baath leadership. He announces that there are plotters, and reads off names one by one. The security guys walk up to those named and take them away.
The US doesn't necessarily need the active cooperation of the Iraqi military. They just need to not fight when the time comes, and be bystanders. I think that's fairly likely; Saddam works by fear, and if everyone recognizes that he _will_ be overthrown by the US, his leverage vanishes. They're not fighting out of belief; once their motivation is gone, they'll stop being a factor. At the end, there will probably be spontaneous efforts to seize power for themselves.
Iran and North Korea might or might not be in the same category. The worst case for the US is a Clauswitzian situation in which the army, government, and people are united. That means the US can't win by simply destroying the top leadership. If the Iranian or North Korean army and people still lend their support to their governments after being decapitated, things would get ugly.