I've been actively opposed to the Iraq war since before it started, although more so in the first couple of years. One of the big reasons that political opposition to the war has never gelled sufficiently to stop it is illustrated by the graphic below from a new USA Today/Gallop poll. While only 40% of Americans fully support the war, opponents have fundimental disagreements on how to actually end it. This has crippled the antiwar effort, and as a result Bush has been able to forestall all attempts to get us out of Iraq.
(Click on image for a clearer version)
The take-home lesson here is that unity is power in politics. A minority can always control the majority if the minority is united and the majority isn't. Unfortunately, achieving a united majority usually requires compromise, something a big part of the active left, including me at times, have a problem with.