The laws of armed conflict are broadly divided into two branches—restrictions placed on the right to initiate war and regulations on the conduct of war. Jus ad bellum determines when military intervention is legal, while jus in bello, outlines the legal behaviour of soldiers on the battlefield. Up until the end of World War II, most emphasis was placed on the development and refinement of jus in bello and its core restrictive principles. Due to the historic level of human and physical destruction caused by the Second World War, along with the onset of the atomic age, attention since 1945 has been principally focused on jus ad bellum restrictions on the right to go to war. The separate standing of jus in bello and jus ad bellum translates into two legal questions for every armed conflict, namely (1) Was the war entered into legally? and (2) Was the conduct of soldiers during that conflict legal? To determine that a state legally entered into a war does not absolve the state and its military from scrutiny over its conduct during the war. For a state to remain within the confines of the laws of armed conflict requires both a legal entry into war as well as the use of force within the parameters established by jus in bello principles.
Jus in Bello
At the onset of the modern international legal system (1648), the core principles that define jus in bello today were already in place, the product of centuries of efforts aimed at reducing the horrors of war. The main jus in bello principles established over time remain in force and are codified in specific international treaties, including the Hague Conventions (1899, 1907), Geneva Conventions (1949) and a series of issue specific treaties dating to the 19th century. Each principle restricts conduct on the battlefield in an effort to humanize and standardized armed conflict. The Geneva Convention and its Protocols are also discussed in Chapter 5.
The first legal requirement placed on soldiers in the field is to differentiate, or discriminate, between lawful targets and unlawful targets. In time of war, soldiers and those who plan the conduct and oversee the behaviour of soldiers are legally required to separate targets into two categories—combatant and non-combatant. Combatants, those who actively take up arms, are legal targets. They may be killed, wounded, or taken captive at any time during the course of the war. Non-combatants, a category that includes civilians, humanitarian relief personnel, journalists, and international observers, among others, may not be targeted. While non-combatants may be detained, restricted or moved, they may not be intentionally injured or killed. The discrimination principle places an enormous burden on soldiers engaged in armed conflict, especially in urban settings. A wilful violation of the principle, however, runs counter to prevailing international law and may constitute a war crime that is punishable by a national or international court.
Prohibition on Unnecessary Suffering
International law allows for the lethal use of force against combatants. Professional soldiers are trained to capture instead of injuring, injure instead of killing, and to kill only as necessary. What soldiers are not allowed to do is inflict injury with the express intent of causing suffering that is unnecessary. The prohibition on unnecessary suffering impacts both the actions taken by soldiers as well as the weapons that are used in armed conflict. Twisting the blade of a bayonet in the leg of an incapacitated enemy combatant for no purpose other than to inflict additional pain and suffering violates the principle. Employing weapons that are designed to cause unnecessary suffering, equally, constitute violations of the laws of armed conflict. Customary international law and treaties have restricted the use of weapons whose sole or primary purpose is to cause suffering. An example of one such prohibited weapon of war is the expanding bullet, which due to its hollow head and soft jacket is designed to expand once it enters the body.
The principle of proportionality requires forces in the field to achieve a balance between the expected level of harm caused to civilians or civilian property and the anticipated military benefit of the proposed attack on a military asset. While it may be legal to target a combatant with a drone air-to-surface attack, those authorizing the operation must take into account the level of civilian harm that will likely be caused by the attack. This is especially the case if the individual being targeted is in the midst of a civilian population at the time of the attack. Proportionality is one of the most difficult jus in bello principles to apply to armed conflict because it requires foresight that may be difficult, if not impossible, to calculate prior to a military operation.
Using deceptive tactics to expose the adversary in armed conflict may constitute an illegal act of perfidy. While outsmarting the adversary is an essential element of war planning and execution, there are limits imposed upon combatants by the laws of armed conflict. False surrender, entering an enemy compound dressed as humanitarian personnel, hiding bombs in children’s toys and feigning non-combatant status, are examples of perfidy and are deemed to be illegal. The perfidy principle is also violated when soldiers intentionally use civilians as shields.
The Martens Clause
Found in the preamble of the 1899 and 1907 Hague Conventions is the Martens Clause, which states that:
Until a more complete code of the laws of war is issued…populations and belligerents remain under the protection of international law, the laws of humanity, and the requirements of the public conscience.
The Martens Clause is designed as an underlying net to legally ensnare violations of the spirit of jus in bello principles that may not be specifically outlawed by custom or treaty. The use of civilian captives for medical experimentation by the Nazis during World War II is one of many examples of an act that is prohibited by the Martens Clause.
In addition to the general restrictions placed on the conduct of soldiers in the field outlined by the jus in bello principles outlined above, states have sought to deem certain weapons of war illegal. International treaties have been drafted to eliminate, for example, anti-personnel landmines (APL)), lasers that cause blinding, cluster bombs, asphyxiating gases, napalm and tumbling bullets. Efforts to ban other weapons of war have yet to reach a sufficient international consensus such that a treaty banning them could be produced and implemented. Attempts to renounce nuclear weapons as illegal have been undertaken for more than fifty years, yet due to objections from the nuclear weapons states such efforts have not produced a legal ban on either the existence or use of nuclear weapons. Depleted uranium shells, which are highly effective in destroying heavily armed vehicles such as tanks, have similarly been shielded from international condemnation by those countries that continue to employ them, despite the probable long-term dangers that the weapons pose to civilian populations.
Jus in bello, its principles and supporting legal documents, have greatly restricted conduct in the field and continue to be revised to take into account the new realities on the battlefield. Attention is now turned towards international efforts to restrict the right of states to enter into conflict, known as jus ad bellum.
Jus ad Bellum
The right to wage war has captivated the attention of policymakers, philosophers, moralists and lawyers for centuries. Up until the drafting of the United Nations Charter in 1945, most efforts since 1648 have resulted in only marginal restrictions on the legal ability of nation-states to enter into war. The Treaties of Westphalia (1648), for example, determined that a religious difference between states was no longer a valid reason for going to war. After the Napoleonic Campaigns concluded, the Concert of Europe (1815) dictated that territorial expansion in Europe required approval from the Concert, which was composed of the five most powerful European countries at the time. Thus, in the years leading up to the First World War nation-states possessed a nearly complete compétence de guerre, or right to wage war. This defining principle of international law at the time was referred to as the Doctrine of Intervention. Except for the few restrictions placed on nation-states in 1648 and 1815, sovereign states possessed the legal right to go to war any time that their leadership concluded it was necessary.
This broad compétence de guerre of states began to erode after World War I. The League of Nations (LON) was empowered to authorize sanctions against states, which threatened international peace and security in violation of its Covenant. The Kellogg-Briand Pact (1928), which was ratified by 65 states, restricted states to the use of force only in response to an armed attack, elevating the concept of self-defence in world politics as the sole legitimate standard for waging war. The failure of the League of Nations to prevent the Second World War, along with the destructive nature of that conflict, lead to a major revision of the laws that regulate entry into war. The United Nations Charter introduced the principle of non-intervention principle in article 2, paragraph 4, which states that:
All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations.
A revolutionary concept, non-intervention nearly eliminated the compétence de guerre of states. Only under the conditions of self-defence, collective self-defence and Security Council authorization could member states of the United Nations (UN) legally enter into war. In 2011, 193 nation-states were members of the United Nations. Since membership in the United Nations requires ratification of the UN Charter, including Article 2, paragraph 4, virtually every state in the international system is bound to the restrictions of the Charter as they relate to the legal entry into war.
Self-defence is a concept that dates to antiquity, holding that nation-states possess the legal right to enter into war in response to an armed attack. When Iraq invaded Kuwait on 2 August 1990, Kuwait possessed the legal right to defend itself. Collective self-defence extends the self-defence to other nations, giving them the legal right to come to the assistance of victim states. The United Nations Charter does not legally require third parties to intervene in response to armed aggression against a victim; however, it provides them with the legal right to do so. As such, not only did Kuwait possess the legal capacity to respond to the Iraqi invasion in 1990 with armed force, so, too, did all other states. Security Council authorization is the third legal justification for entering into war. The Security Council, a permanent organ of the United Nations, is composed of fifteen member states. Five of those members—China, France, Russia, United Kingdom, United States—are permanent (P5), while the remaining ten Council members are present for two-year appointments. In addition to a permanent presence on the Security Council, the P5 enjoy veto power over any Council resolution. At any time the Security Council decides that one nation’s invasion of another constitutes aggression, it may authorize any or all UN member states to go to war on behalf of the victim state. As such, when Iraq invaded Kuwait in 1990, automatically Kuwait possessed the legal right to militarily defend itself (self-defence) and all other UN member states enjoyed the right to come to its defence (collective self-defence). When the Security Council authorized force against Iraq to liberate Kuwait, the third (and separate) legal recourse to war was extended to states to go to war against Iraq. 
The willingness of the UN Security Council to authorize war has varied greatly since the organization’s inception. Only twice has the Council explicitly authorized war in response to an armed invasion, with both instances of Chapter VII war authorization happening under unique circumstances. In 1950, the Council deliberated the Korean War without the Soviet representative present, removing the veto from the calculation. Later, when the Security Council convened to consider expanding its original war mandate to include the reunification of the Korean peninsula, the Soviet representative returned to the Council and vetoed the effort. In 1990, the Security Council addressed the Iraqi invasion of Kuwait during an historic period of cooperation between the superpowers resulting from the peaceful end of the Cold War. The result was an authorization for war and a diverse international military coalition that forced Iraq’s military from Kuwait. Despite numerous opportunities during and after the Cold War for the Security Council to take action in response to inter-state conflict, in most instances it either declined to do so or was unable to as a result of the veto. As such, Security Council authorization for war historically has been the exception rather than the rule.
The legal right of states to go to war, therefore, has been dramatically and qualitatively reduced by the revisions to jus ad bellum introduced in 1945 by the United Nations Charter. No longer may states legally attack other states except under the narrow confines outlined in the Charter. The purpose of the Charter’s restriction on the legal right of states to go to war is to reduce the indiscriminate use of force. The fact that states continue to opt for war without legal cause, including waging undeclared wars, indicates their ability to sidestep the Charter’s restraints.
The laws that regulate international conflict are in constant need of revision and updating as the global environment changes. Technological innovations create situations that traditional international law struggles to fully address. This is the case with both jus ad bellum and jus in bello rules. For example, Article 51 of the United Nations Charter authorizes states to use force in self-defence when an armed attack occurs against the state. Cyber attacks, which can cause devastating consequences for the targeted state, do not technically qualify as an ‘armed attack.’ The first ‘cyber war’ occurred in 2008 when Russia attacked the financial infrastructure of Estonia, crippling the country’s banking and credit system.
Jus in bello principles face equally daunting challenges on the 21st century battlefield. Robotic soldiers, and other forms of Automated Robotic Killing Machines (ARKM), attack drones that are controlled through satellite feeds, and the use of sound waves and other non-lethal defensive weapons are minimally regulated by existing treaty law. In order to maximize its role in the regulation of armed conflict, international law must quickly and consistently evolve to apply to new circumstances on and off the battlefield.