In September of 2001, a group of Islamic radicals flew airplanes into major landmarks in the United States. Thousands of innocent people were killed.
The United States decided to attack the leaders behind these acts of war in Afghanistan. The United Nations approved this invasion and NATO gathered around the US in undertaking this mission.
Shortly thereafter, with information provided by several different security and intelligence services, the United States with the approval of both Houses (and with the support of Senators John Kerry and Hillary Clinton) decided to invade Iraq to get rid of notorious despot Sadaam Hussein who was reported to have weapons of mass destruction. Seeing as he had used such against his own people, this seemed to be a reasonable action. The United Nations refused to approve this mission.
What was the difference?
Approval of the United Nations.
Could this have been influenced by the Iraqi oil that France and German had become dependent on?