The United States Army used 1,100 M1 Abrams in the course of the Iraq War. They proved to have an unexpectedly high vulnerability to improvised explosive devices. A relatively new type of remotely detonated mine, the explosively formed penetrator, was used with some success against American armoured vehicles. However, with upgrades to their rear armour, M1s proved to be valuable in urban combat; at the Second Battle of Fallujah the United States Marines brought in two extra companies of M1s. Britain deployed its Challenger 2 tanks to support its operations in southern Iraq.