True, but those are politicians. It's normal for that breed of people to go against what the population wants. It's already cost 1 government their rule, in Spain the war was at least partly responsible for a major shift...
And if you're specifically referring to dutch leaders, two major parties were in favor, and a whole bunch of others were not at all supportive of the war. But since the governing coalition as a whole made a compromise to enter the war (probably under that threat of "if you're not with us you're against us" and some economic pressure) our troops are now there as well. It's still debated wether or not they will stay past July.
France's scepticism was well founded in the end: No weapons found... And had the evidence been so overwhelming, going public with it would have got the public behind the US. With that kind of support even Chirac would have given in. Problem: THERE WAS/IS NO SUCH EVIDENCE.
A slow one I'll grant you. And the UN is not the SC. But as an arbiter the US by itself is also not qualified... One cannot be judge, jury and executioner and state the trials were fair. And the US did assume all those rules in the end.
So what's next? Pakistan for instance is known to have nukes... isn't that country (known for fundamentalist sympathies, AQ training facilities near the borders with Afghanistan) a liability too? Will you invade it to prevent "accidents" where AQ gets it's hands on Pakistani nukes? Or what of the many African countries in total chaos? Who knows what sick twisted little people are doing out there?

If the US only has to say "we know you have something you should not have", not prove that, and invade... then I fear international law has seen it's end. Which means a new arms race is the next logical step...