Worst Case Scenario A:
1. Hussein Kamil doesn't order the WMD destroyed in 1995. Iraq actually does try and build a concealed program that somehow gets past the UN inspectors and the US.
2. When the US and Coalition go in, Hussein launches the WMDs at the troops, plus Israel, Kuwait, Bahrain and the UAE. Thousands of troops die. Even more civillians die (especially the ones not in Israel, who don't have plans for WMD attacks on civillians).
3. The US goes nuclear. Iraq is glassed. Millions die. The world is upset, and the mideast even more so. (After all, they didn't side with Iraq, but they got the fallout...)
Worst Case Scenario B:
1. Riots break out in Jordan, Bahrain and the UAE over their countries covert support for the Iraq War. (No riots in Kuwait- after all, they hate Saddam more than anyone besides the Kurds!)
2. Al Quaeda takes advantage and topples the governments.
Worst Case Scenario C:
1. The US (or UK), believing that WMD were used or about to be used use nukes on Iraq.
2. Iraq invokes the Non-Proliferation Treaty and asks for help.
3. China or Russia insist on proportionate retaliation (a la "Fail-Safe") and retaliate.
4. The US and UK nuke back-full force.
5. WWIII.
Worst Case Scenario D:
1. As Coalition forces prepare for invasion, Iranian forces go on alert and mass at the border.
2. When the Coalition goes in, so does Iran, claiming (truly or falsely) that they were invited in by Iraqi opposition groups.
3. Iranian troops entering Iraq wind up exchanging fire with Coalition forces (not to mention Iraqi forces and Kurdish peshmerga...).
4. Shi'ites rise up against Saddam and the Coalition in Iraq.
5. Iran attacks the US bases in the Gulf area, in addition to its fleet.
Worst Case Scenario E:
1. Iraq appeals to OPEC to not sell oil to the US and UK in the event of an invasion. OPEC agrees.
2. As soson as the trops cross the border, OPEC ceases selling oil to the US and UK. Gas prices skyrocket.
3. The UK is able to survive, but the skyrocketing Gasoline prices push the US into Recession.
Worst Case Scenario F:
1. As the US troops are in the process of invasion, Taliban forces step up attacks on NATO forces in Afghanistan.
2. Around the same time, North Korean forces cross into South Korea, calling for reunification of the Korean Peninsula by force.
Worst Case Scenario G:
1. The lack of WMD in Iraq causes widespread public protests in the Coalition countries. Opposition groups take to the streets. Anger mounts.
2. Some disgruntled protestors begin calling for action by force.
3. A number of war supporting politicians are assasinated.
4. Countries declare martial law.
5. Declaration of martial law polarizes society. Second US and UK Civil Wars begin (And First Australian, Second Spanish, Nth Polish, First Georgian, etc?)