(Captain's log): Let's do some supposing.
Suppose we (the UK and US) do ignore all the pressure and last-minute finagling and do actually attack Iraq, which I think now is virtually certain.
Suppose we win, which is absolutely certain.
And suppose, once we've done so, and have occupied Iraq and have full (really full, not UN full) access to Iraq's records and can truly find what they have, that we find that everything we've been saying about their WMDs is really true; that they have chem and bio weapons and banned delivery systems, and are near to developing nukes, which I also think is extremely likely.
One more and the most important: suppose that the records also show that during the 1990's companies in France or Germany (or both) actively and deliberately broke the sanctions and sold equipment and supplies to Iraq which helped it to create these things, and that the governments of Germany and France knew and approved of this and actively helped. That's the biggest and most speculative suppose.
On that I can't place a probability; there's no way of knowing right now whether this happened, or whether such records will be found. But I don't consider the possibility of this to be vanishingly small. I think the chance is decent that some such illegal sales to Iraq took place, but I can't say how likely it is that the governments there actively approved of it, or at least deliberately ignored it (which is bad enough). That's the wild card.
But we're supposing now, and so what we've supposed is that after we conquer Iraq we come into possession of undisputable proof of treachery by the German and/or French governments, who are supposedly our allies. What should be done in that case? What result would this have?
The first question our governments would face is whether to reveal it. There's a case to be made for keeping it secret and using it for blackmail. (Which is why I will become extraordinarily suspicious if there's a notable change in tone and behavior from either or both nations about two months after the war ends.)
But if such information existed and were revealed, either deliberately or because it couldn't be concealed, then what?
I think at that point that anything resembling formal alliance would have to end. The degree of fury this would cause in the American people should not be underestimated, and it would become politically impossible for the US government to continue to treat either nation in a friendly manner. Our relations with them would come to resemble those we have with China if not being worse.
Either the US would formally pull out of NATO or else issue an "either they go or we do" ultimatum to get Germany and France ejected. And all American forces in Germany would leave as soon as they possibly could. (Such forces as we kept in Europe would probably move to either Poland or the Czech Republic, both of which have expressed interest in hosting them.)
NATO without the US is a meaningless joke, and everyone knows it, especially since the UK would almost certainly also leave, and they have the second most powerful military in the alliance. But it's not clear the alliance could survive anyway when it turned out that members had done that kind of thing against one another. And enthusiasm by eastern European nations about joining NATO would fizzle. So I don't see how NATO would survive in any meaningful sense.
It seems unlikely that this would lead to an immediate and direct war with the US, though formal diplomatic relations would obviously go into the toilet as a result. And even without any formal government action, I think that the European companies involved would find it difficult thereafter to make any business deals in America, and this would cause quite substantial economic havoc, here certainly, but particularly in Europe. In fact, there would be a de facto economic boycott here of the majority of German and French products (and to a lesser extent of other European producers as well, as a practical matter) as the direct result of millions of citizens making their own choices, whether the US government declared formal trade sanctions or not. (Which means that the big winner would be Japan, whose trade with the US would rise. This might well be enough lift them out of their economic malaise and give them the slack to work on the underlying structural problems in their economy.)
This would also really hurt the tourist industries in both nations. That happened to some extent after 9/11 because people feared to fly, but this would be worse and very long lasting, and that too would contribute to a general downturn in their economies. You can basically forget about any significant number of Americans traveling to either nation and spending their dollars there. What you're looking at is the distinct possibility of Europe's economy imploding as a result of cascading failures.
And if it were shown that at least one UNSC veto power had been violating UNSC-approved sanctions, the UN is dead. Just as with NATO, the UN without the US is an empty joke and the majority of the people of the US would never again support it.
A different question would be what political effect this would have on the EU. How would other nations in the EU react to this kind of perfidy? Certainly it would mean that the UK would become far less likely to integrate, but it would surely also affect the attitudes in Italy and Spain and the Netherlands.
The biggest question, and the one I'm least able to