Just War is a concept that governs the conditions under which a war can be waged and how it can be waged. Increasingly, it also handles the post-war issues of legal claims about war crimes committed previously. The theory of Just War is known in variant forms since antiquity and several versions exist within both religious and philosophical contexts. Today, there is an international regime which governs war based on a constitution (the Geneva Convention), a legislative body (international meetings and regular sessions of the United Nations Security Council) and a judicial system (the International Criminal Court). Some say the system is too weak, seeing it as having no ability to enforce its decisions on unwilling states or non-state actors, arrest indicted world leaders, or launch police operations in order to ensure humanitarian intervention.
Originally, the theory stood as a doctrine developed by highly influential Catholic thinkers and Church leaders who dealt with the issues of realms within Christian Europe fighting each other. Some of the key principles were developed and advocated by Thomas Aquinas. Later, as the Church lost influence over European and international politics, international political and legal institutions subsumed the authority of the church in governing war.
Just War Theory as termed in English typically encompasses three Latin-named concepts governing different aspects of defining a so-called "just war."
Jus ad bellum considers the right to go to war. Typically, this weighs whether or not a clear danger to innocent life is present against a population, in conjunction with whatever causes might develop hostile relations between two states. Another factor is whether or not the state or government itself has the proper political and judicial institutions to maintain public order and enforce fair courts. This criterion can be controversial because the delegitimization of regimes is a typical tool of propaganda emanating from a hostile state. Additionally, ethnic conflicts have seen attempts to undermine the right of another entity to exist (for example, Israel vis-a-vis its Arab neighbors), creating a slippery slope where one state could justify for itself and a significant minority of allies a war where the objective might otherwise be illegitimate. The war must also have some probability of success so as not to waste human life, must be weighed for an acceptable intention (such as correcting wrongs done and not for the acquisition of resources or territory), plus should only be a last resort relative to some diplomatic or judicial effort made earlier.
Jus in bello considers how the war ought to be conducted. This is a separate issue from whether or not a war can be launched in the first place, and the decision to go to war arguably should or should not be considered an element of jus in bello as well, which arguably might render any conduct during a war illegal and thus criminal. Aside from the right or not to go to war, a force must limit itself to military objectives and only allow for the deaths of non-combatants if they are unavoidable. In conjunction with these facts, the principle of proportionality, where the (collateral) damage inflicted by an attack is minimal compared to the strategic necessity and gain of the attack. Any situation where such damage and the casualties (non-combatant, though some might argue also combatant) are deemed unnecessary would violate this principle. An example of a debate about the observance of this principle comes in the debate about the State of Israel's wars in Lebanon and the Gaza Strip since 2006, where critics have been quick to accuse the country of disproportionate use of force (indiscriminate bombing in civilian-populated areas, use of unconventional weapons), while Israel and supporters of its position argue the accusations were made in a knee-jerk fashion, enemy combatants have been operating in close quarters to civilians, and any injury from an unconventional weapon was collateral in that it served some other purpose (white phosphorous being used for lighting dark areas versus its use as an incendiary weapon). Additionally, the assessment of a target's military necessity and the subsequent fair treatment of prisoners of war must be maintained.
The third concept, Jus post bellum, points to the need for justice during the final stages of a war and the transition to peace time.
The original, religious and particularly Catholic interpretation of the justification for war has been maintained in some intellectual circles independent of the international legal structure. Paul J. Griffiths and George Weigel deal with the Catholic acceptance of condemnation of the 2001 invasion of Afghanistan, where the United States responded to the September 11, 2001 attacks. They argue that the acceptance of the government's version of the story does not justify the launching of a war and that more concrete evidence that the Taliban was involved should have been demanded to justify the war.
Today, many arguments revolve around the 2003 war in Iraq where the United States failed to gain official support for an invasion then retroactively justified it on earlier U.N. resolutions. Many have called this a mocking of the international system and regarded it as having damaged the concept of humanitarian intervention later when it was accepted Saddam Hussein posed no international threat as a producer of unconventional weapons.