The Bush Administration was, indeed, mildly fixated on Iraq from Bush's first day in office. This fixation had a legitimate basis in that Iraq was the only nation at that time that the US was engaging in any sort of military action against (enforcement of the no-fly zones). Nonetheless, although the basis was legitimate, the attention paid to Iraq was disproportionate, mostly because of the relentless advocacy of Paul Wolfowitz, the neoconservative deputy secretary of defense, and Dick Cheney's sense of unfinished business from the 1991 Gulf War, when he was serving as secretary of defense.
Donald Rumsfeld, the secretary of defense, upon taking office, found all the war plans to be in disarray. War planning is important; it allows the military to respond in the most effective way to any given situation because many of the contingencies are thought through up front, minimizing loss of life and maximizing the military's ability to fight and win the conflict. The necessity of current war plans is entirely independent of a need or desire to execute them.
What Rumsfeld discovered is that many of the war plans dated back to the mid-1990s, and that furthermore the war plans did not have their underlying assumptions identified in any way. For example, the war plan for the Korean peninsula (one of the more fully-formed plans, for what I hope are obvious reasons) did not make it clear whether it assumed that North Korea had nuclear weapons. The Iraq plan (also a fully-formed plan, as was right and proper) had similar difficulties. Rumsfeld, shortly after taking office, ordered rapid analysis and freshening of all the war plans, including Korea and Iraq, and in this act he did a very good thing that desperately needed doing.
The bona fide need to revise the Iraq war plan converged with the ideological desire in the Administration to do something about Saddam. As early as December 2001, with the war in Afghanistan going well but still very much underway, Bush requested briefings on the Iraq war plan for his war cabinet (which consists of Cheney, Rumsfeld, Colin Powell, Condoleeza Rice, and the CIA director, at that time George Tenet). The plans had some prerequisites that were themselves prudent regardless of whether war would ultimately occur, such as moving the alternate air command and control center into Qatar from Saudi Arabia, and the decision was made to implement such prerequisites immediately. Note, though, that the only reason Iraq was at the very top of the agenda in the first place was due to the initial pre-9/11 desire within the Administration to depose Saddam.
More information in Part 2.