Though United States provided aid and support to Afghan Mujahideen, Pakistan and, Saudi Arabia since the moment Soviets intruded Afghanistan in the form of arms supply and all that stuffs, but US forces entered Afghanistan on October 7, 2001 following the WTC attack on 9/11/2001...... The soul target of the invasion was to hunt down Osama Bin Laden who still manages to stay alive and his location unknown.....
