The legality of a targeted drone strike must be evaluated in accordance with international humanitarian law (IHL), including the fundamental principles of distinction, proportionality, humanity, and military necessity. The part of The Charter of the United Nations that regulates "action with respect to threats to the peace, breaches of the peace, and acts of aggression" is Chapter VII (articles 39–50), which requires that it is the Security Council that determines any threat to peace and decides on measures to be taken to maintain or restore peace. Article 51 mentions the only exception, as being members of the United Nations have "the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security". Targeted killing operations, according to Harvard Law School professors Gabriella Blum and Philip Heymann, amplify the tension between addressing terrorism as a crime versus addressing terrorism as an act of war. Governments pursuing a law enforcement strategy punish persons for their individual guilt, which must be proven in a court of law, where the accused enjoys the protections of due process guarantees. Governments in the midst of war, on the other hand, may claim a legal obligation to take advantage of the relaxation of peacetime constraints on the use of deadly force. Enemy combatants may be targeted and killed not because they are guilty, but because they are potentially lethal agents of a hostile party. No advance warning is necessary, no attempt to arrest or capture is required, and no effort to minimize casualties among enemy forces is demanded by law. The tactic raises complex questions as to the legal basis for its application, who qualifies as an appropriate "hit list" target, and what circumstances must exist before the tactic may be employed. Opinions range from people considering it a legal form of self-defense that reduces terrorism, to people calling it an extrajudicial killing that lacks due process, and which leads to more violence. Methods used have included firing a Hellfire missile from an AH-64 Apache attack helicopter (Israel), or a Predator or Reaper drone (an unmanned, remote-controlled plane), detonating a cell phone bomb, and long-range sniper shooting. Countries such as the U.S. (in Pakistan and Yemen) and Israel (in the West Bank and Gaza) have used targeted killing to kill members of groups such as