Introduction The war in Afghanistan has been discussed for several years to determine whether the use of force was just according to the just war theory. The United Nations defined and set the standards for a just war at Nuremberg. The standards of a just war are remunerated and explained in articles 2 and 51 of the United Nations charter (Byers 2007). According to the UN charter, the seven principles of a just war include a just cause, adherence to international law, success should be possible, the means of combat must be proportionate to the desired results, it should be the last resort after failure of other conflict resolution means, and the war must have been proclaimed by a lawful authority. The use of force by the U.S. was just under the just war theory because it was executed for self-defense and elimination of a terrorist threat that was difficult to combat. The just war theory The main reason why the U.S waged war on Afghanistan in 2001 was because of the 9/11 attack that led to the demise of many Americans. The war had a just cause because the U.S government was protecting its citizens by combating acts of terrorism that were threatening to violate the human rights of Americans and destabilize their lives (Byers 2007). The intention of waging the war was to eradicate the threat of international terrorism that Al-Qaeda posed through their attacks. It is illegal for a sovereign state to attack a non-state entity through combat as was the case with Al-Qaeda. However, the war was inevitable because the terror group had infiltrated the leadership of Taliban and posed great threats to American security (Duffy 2005). In addition, the Afghan government had failed to stop the activities of the group. The U.S acted in self-defense. A state is considered to be involved in terrorism if a terrorist group comprises one of its organs, if the state tolerates terrorist activities, if the state supports the activities of the group or if the state is unable to address the problem of terrorism within its borders (Gazzini 2006). The Afghan government was unable to stop the activities of Al-Qaeda and therefore the U.S. had to intervene (Duffy 2005). The United States used forced out of necessity because the local government had exhausted all means of combat and had not succeeded in stopping the group. Another requirement of the just war theory is the use of force that is proportionate to the outcome desired. The use of massive force against Afghanistan was imperative because the terrorist groups were very strong to combat and were receiving support from the government (Gazzini 2006). Force was used in order to avoid the risks of further attacks that the terror group was planning to launch. Moreover, excessive force was used from the perspective of self-defense because of the risk of potential future attacks against the U.S. The 9/11 attack and threats of further attacks necessitated the use of force in order to annihilate the activities of the group completely. The use of excessive force can be supported by using the argument of self-defense that is allowed as a reason to wage war by the United Nations Security council (Tams 2009). Terrorism is hard to curb because its consequences are irreversible, instantaneous, unexpected, severe, and sophisticated. The uncertainty of attacks makes it d