Why did the anti-war movement fail?

With the disgraceful fifth anniversary of the War on Iraq rapidly approaching, it is a good time to examine why we all failed to stop this war when the vast majority of people in the UK, Europe and even the US was dead set against it. What happened that two million people could march in London on February 15th and yet the war started the very next month?

Which is why I set up an open thread over at Prog Gold, to discuss this question. If y’all would like to hop on over and give your opinion on this matter?