International law is the set of rules regarded and accepted in relations between nations. It serves as a framework for the practice of stable and organized international relations. International law differs from state-based legal systems in that it is applicable to countries rather than to individual citizens. National law may become international law when treaties permit national jurisdiction to supranational tribunals such as the European Court of Human Rights or the International Criminal Court. Treaties such as the Geneva Conventions may require national law to conform to respective parts. National laws or constitutions may provide for the implementation or integration of international legal obligations. International law is consent-based governance, as there is no means of enforcement in a world dominated by sovereign states; this means that a state may choose to not abide by international law, to break its treaty. However, violations of customary international law and peremptory norms can lead to military action or other forms of coercion, such as diplomatic pressure or economic sanctions.
The current order of international law, the equality of sovereignty between nations, was formed through the conclusion of the "Peace of Westphalia" in 1648. Prior to 1648, on the basis of the purpose of war or the legitimacy of war, it sought to distinguish whether the war was a "just war" or not; this theory of power interruptions can be found in the writings of the Roman Cicero and the writings of St. Augustine. According to the theory of armistice, the nation that caused unwarranted war could not enjoy the right to obtain or conquer trophies that were legitimate at the time The 17th, 18th and 19th centuries saw the growth of the concept of the sovereign "nation-state", which consisted of a nation controlled by a centralised system of government; the concept of nationalism became important as people began to see themselves as citizens of a particular nation with a distinct national identity. Until the mid-19th century, relations between nation-states were dictated by treaty, agreements to behave in a certain way towards another state, unenforceable except by force, not binding except as matters of honor and faithfulness.
But treaties alone became toothless and wars became destructive, most markedly towards civilians, who decried their horrors, leading to calls for regulation of the acts of states in times of war. The modern study of international law starts in the early 19th century, but its origins go back at least to the 16th century, Alberico Gentili, Francisco de Vitoria and Hugo Grotius, the "fathers of international law." Several legal systems developed in Europe, including the codified systems of continental European states and English common law, based on decisions by judges and not by written codes. Other areas developed differing legal systems, with the Chinese legal tradition dating back more than four thousand years, although at the end of the 19th century, there was still no written code for civil proceedings. One of the first instruments of modern international law was the Lieber Code, passed in 1863 by the Congress of the United States, to govern the conduct of US forces during the United States Civil War and considered to be the first written recitation of the rules and articles of war, adhered to by all civilised nations, the precursor of international law.
This led to the first prosecution for war crimes—in the case of United States prisoners of war held in cruel and depraved conditions at Andersonville, Georgia, in which the Confederate commandant of that camp was tried and hanged, the only Confederate soldier to be punished by death in the aftermath of the entire Civil War. In the years that followed, other states subscribed to limitations of their conduct, numerous other treaties and bodies were created to regulate the conduct of states towards one another in terms of these treaties, but not limited to, the Permanent Court of Arbitration in 1899; because international law is a new area of law its development and propriety in applicable areas are subject to dispute. Under article 38 of the Statute of the International Court of Justice, international law has three principal sources: international treaties and general principles of law. In addition, judicial decisions and teachings may be applied as "subsidiary means for the determination of rules of law", International treaty law comprises obligations states expressly and voluntarily accept between themselves in treaties.
Customary international law is derived from the consistent practice of States accompanied by opinio juris, i.e. the conviction of States that the consistent practice is required by a legal obligation. Judgments of international tribunals as well as scholarly works have traditionally been looked to as persuasive sources for custom in addition to direct evidence of state behavior. Attempts to codify customary international law picked up momentum after the Second World War with the formation of the International Law Commission, under the aegis of the United Nations. Codified customary law is made the binding interpretation of the underlying custom by agreement through treaty. For states not party to such treaties, the work of the ILC may still be accepted as custom applying to those states. General principles of law are those recognized by the major legal systems of the world. Certain norms of international law achieve the binding force of peremptory norms as to include all states with no permissible derogations.
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