http://www.amazon.com/Cobra-II-Inside-Invasion-Occupation-ebook/dp/B000GCFCK0
Page 13-14 "Once the two men [Clinton and Bush] were behind closed, Clinton told Bush that he read his campaign statements carefully and his impression was that his two priorities were national missile defense and Iraq. Bush said this was correct"
This was in December 2000. Of course keep in mind the US maintained air patrols in Iraq and had since the Gulf War. The book also details how the Iraq invasion was on the president's mind even shortly after 9/11.
This is a really deep dive question to be honest. Here's my take at a really basic answer...
For quite a while now, DoD has practiced GFM (Global Force Management) which basically supplies forces to CCDRs (Combatant Commanders) in a very regimented and scheduled fashion. Operationally, services either align their forces with certain Geographic Combatant Commands or retain them (service-retained forces). The services all receive operational requirements that are codified by DoD via the SecDef's Orders Book (SDOB), and the services must then fulfill those requirements with their available forces, but these requirements are pretty much always determined/requested at the CCDR or national level. Very basically though, this is where you can begin to find order and patterns in rotational deployments. For example, I MEF was responsible for SPMAGTF-CR-CC while II MEF was responsible for SPMAGTF-CR-AF.
When you consider all the various requirements that an entire service may have, it can be like a Tetris game to place everything in the right spot in order to make everything else work - especially when you tie in pre-deployment and support requirements that place constraints on time. Deployment-to-dwell ratio is an additional significant variable.
The folks doing the bulk of this work at the MEFs and MSCs are 0505 majors working for their respective G-3s with the CGs sitting as the decision-makers.
The deliberate operational requirements - the rotational ones - are the easiest parts of that puzzle. Those can have units slated to them years in advance. Even SLTEs are slated years out. However, when you have emerging operational requirements - say for example an earthquake in Haiti, a tsunami in the Pacific, or pandemic support within CONUS - it can toss a wrench into things. Depending on the requirement, the command may need to take a unit off the ready bench because you need your best-trained force and you need it now. However, that may cause other requirements to be cancelled or require a replacement force. So, one day your unit is training for a UDP rotation to Okinawa or ITX in the summer, and the next minute you're switching gears to go protect a hospital ship or conduct HA/DR.
Hopefully that word salad maybe provides a little bit of enlightenment. If you're interested in a bit of relevant recent history and how all of this works in reality, go check out the first few chapters of the book Cobra II.