One big question you would have to ask is how the use of nuclear weapons would play with the populations of current members of the coalition.

If the US starts using Nuclear weapons in a confrontation, many of the undecided in the UK will get pushed over into the anti war camp very quickly. This could be to the extent of forcing the UK government to withdraw from Iraq. at one stroke the US will have increased the area of battle, and reduced the forces available to fight.

Any idiot can face a crisis - it's day to day living that wears you out.