al-Sadr is a very open anti-American idealist. He makes no bones about it. His troops, the Mehdi Army, fought some of the most fierce battles against American troops during the Iraq war. Early on in the war al-Sadr was very vocal and raising quite the stink with our troops. As things heated up for him he hid and then waited for right time. That time is now. With the with drawl of American troops, he immediately resurfaced and began to call for the dissolution of the government that is in place now. He is demanding "new" elections.
He is powerful and has many followers. According to his family lineage he is a direct decedent of Muhhamad. He hates America and that should be a concern to all. The US military had the chance to take this guy out years ago and we didn't. I think that is a move that is going to haunt us in the long run. There is an old quote, "those who don't learn from history are destined to repeat it"; bin Laden and Hussein to name a few examples.