Because, as the Joint Chiefs outlined in the aftermath of the end of the Cold War, the United States shifted from global containment to confronting regional powers. After Iraq's victory vs Iran in 1988, it had a large military (still 400,000 or so troops even after post-war cutbacks), had shown that it could defend itself against a powerful adversary, and was beginning to assert itself in Arab politics. It wanted to created an Arab Cooperative Council, with Egypt, Yemen and others, as an independent political bloc in the region. The US has never tolerated the existence of regional hegemons. It fought both Japan and Germany to prevent the rise of regional hegemons. It now threatens China and Russia for the same reason. The presence of a militarily powerful, independent Iraq was not acceptable. With Iraq defeated, the US could control the energy resources of the Gulf. The US had long stated its goal to do so, since the days when oil was discovered in Saudi Arabia.