The 2003 invasion of Iraq was not a spontaneous thing, at least not when it came to planning for it. Throughout the 1990s, the U.S. Department of Defense studied their options about how to deal with Iraq. President Bill Clinton ordered up these plans, as well as a series of bombing attacks, in December, 1998 (Operation Desert Fox). This was done  to try and force Iraq to abide by the agreement it had signed at the end of the 1991 war. Clinton pointed out, at the time,  that Iraq was still a threat to its neighbors, and the world, because UN inspectors were blocked from many sites suspected of supporting the development of nuclear, chemical and biological weapons. Two years earlier, two of Saddams senior generals (and both sons-in-law of his) had defected, and revealed continuing research on weapons of mass destruction. However, nothing much happened after 1998, until 2003. 

Clinton, noting the failure of the 1998 air attacks, ordered up plans for more vigorous operations against Saddam. These, however, were never acted on because Kosovo emerged as a critical situation in 1999 (because the Serbs were driving out the entire Albanian population of over a million people). Our NATO allies were unable to deal with this themselves, the UN refused to condone a military intervention, and the Europeans leaned on Clinton to send in American troops to take care of the situation. 

The post Desert Fox plans are classified. However, it is known that American Special Forces frequently operated in Kuwait, that the CIA had developed some agents inside Iraq, and the Department of Defense had created the capability to quickly get several divisions of ground troops, and hundreds of warplanes, to the region, on short notice. It was this capability that made it possible to put together the 2003 invasion force on such short notice. 

Typically, these plans stay classified for a long time. If you reveal the details of those plans, you tell potential enemies a lot about your military capabilities. You also scare the hell out of friends, and enemies, in the vicinity. Thus the secrecy. The purpose of these war plans is to prepare, not give aid to potential enemies, or make allies nervous.