some rules to an objective, independent "natural law." These jus gentium ideas of fairness and natural law have survived and are reflected in modern international law. Early Islamic law's principles concerning military conduct and the treatment of prisoners of war under the early Caliphate are considered precursors to international humanitarian law. The many requirements on how prisoners of war should be treated included, for example, providing shelter, food and clothing, respecting their cultures, and preventing any acts of execution, rape or revenge. Some of these principles were not codified in Western international law until modern times. Islamic law under the early Caliphate institutionalised humanitarian limitations on military conduct, including attempts to limit the severity of war, guidelines for ceasing hostilities, distinguishing between civilians and combatants, preventing unnecessary destruction, and caring for the sick and wounded. Nation-states After the fall of the Roman Empire and the collapse of the Holy Roman Empire into independent cities, principalities, kingdoms and nations, for the first time there was a real need for rules of conduct between a large international community. Without an empire or a dominant religious leadership to moderate and direct international dealings, most of Europe looked to Justinian's code of law from the Roman Empire, and the canon law of the Catholic Church for inspiration. International trade was the real catalyst for the development of objective rules of behaviour between states. Without a code of conduct, there was little to guarantee trade or protect the merchants of one state from the actions of another. Economic self-interest drove the evolution of common international trade rules, and most importantly the rules and customs of maritime law. As international trade, exploration and warfare became more involved and complex, the need for common international customs and practices became even more important. The Hanseatic League of the more than 150 entities in what is now Germany, Scandinavia, and the Baltic states developed many useful international customs, which facilitated trade and communication among other things. The Italian city-states developed diplomatic rules, as they began sending ambassadors to foreign capitals. Treaties—agreements between governments intended to be binding—became a useful tool to protect commerce. The horrors of the Thirty Years' War, meanwhile, created an outcry for rules of combat that would protect civilian communities. Hugo Grotius International practices, customs, rules and treaties proliferated to the point of complexity. Several scholars sought to compile them all into organized treatises. The most