From an article on Wikipedia, Protests against the Iraq War
According to the French academic Dominique Reynié, between January 3 and April 12, 2003, 36 million people across the globe took part in almost 3,000 protests against the Iraq war.
The article lists major demonstrations against the war, prior to it even beginning. On September 24, 2002, betweeen 150,000 and 400,000 marched against the war in London. On October 26, 100,000 in Washington, DC, and 50,000 in San Francisco.
And the war proceeded, right on schedule.
Protests, by themselves, are not enough. Total loss of confidence in the government is not enough. There actually has to be a popular alternative, with sufficient organization to bring it to life.