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 behavior 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 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.
Read more about this topic: History Of Public International Law