Technically, WMDs was only one of a number of casus belli stated officially by the US prior to the war, which included Iraq's non-compliance with the terms of the 1991 ceasefire, its continued hampering of UNMOVIC weapons inspection teams, its repression of its civilian population, its attempted assassination of a former US president, its continued firing on US aircraft enforcing the no-fly zone and its harbouring of international terrorists. I don't believe the US and UK should have launched that war in the first place and I personally thought the rationale was a little shaky, but the historical record does show that it wasn't only about WMDs.

The legal basis for the continued US presence is well-established and widely known, so I'm surprised you feel confident enough to dare anybody to explain it to you. Subsequent to the invasion, the presence of US and other coalition forces in Iraq was legitimised by, in turn, UNSC Resolution 1483 (2003), 1546 (2004), 1637 (2005) and 1723 (2006), IIRC. The last action taken by the UNSC with regards to multinational forces in Iraq was to extend the mandate authorising their presence till December 2008. Thereafter, the continued presence of US forces in Iraq has been legally authorised by the US-Iraq Status of Forces Agreement, which calls for US forces to leave the country by December 31, 2011. Whatever the moral case for a continued US presence in Iraq may be, the legal basis is sound.

Something else that's worth noting is that the Wikileaks video dates from 2007, during the height of combat operations surrounding the surge. At the time those sorts of engagements were a daily occurrence. This is no longer the case; these days the remaining US forces in Iraq are involved in relatively few direct combat operations and the overall level of violence in the country has dropped to a level where a slow US withdrawal is now possible without causing a security vacuum. From a US perspective Iraq is now a solved problem and the war is just about over. I must give credit to General Petraeus and his sane and unorthodox counter-insurgency strategy for that, for what it's worth.

So the stuff shown in the Wikileaks footage seldom happens in Iraq anymore. I would suggest that your time is better spent not impotently demanding that US forces leave now based on your reaction to a three year old video, but rather applying pressure on your representatives to ensure the planned withdrawal from Iraq happens on time and that US combat operations in that country end as scheduled on 31 August. I see no reason why it won't though, and in my opinion Afghanistan is a far more pressing concern on which to focus at the moment.