The True Majority is a progressive, activist organization.
One of the groups supporting the Mandate for Peace is the Iraq Veterans Against the War.
Anyway, given my early childhood history with the Vietnam Veterans Against the War, I am interested to learn about the IVAW. The IVAW seems to be small right now, but hopefully since it’s now permissible to disapprove of the war, the group will grow.
They have a great list of reasons to be against the war: http://www.ivaw.org/faq. They also provide a rationale for immediate withdrawal. It’s an interesting read.