The History of International Law Timeline (open access from Oxford Academic) is a concise map of 'the broad history of public international law with particular attention paid to the signing of major treaties, the foundation of fundamental institutions, the birth of major figures in international law and milestones in the development of some of the field’s best-known doctrines'. The timeline begins with the Treaty of Tordesillas in 1494.
This collection is an invaluable collection of legal treatises, historical records and works from the 17th century to the beginning of the 20th century. Providing broad coverage, the collection enables detailed research on centuries of major events of legal history, from public policy and taxation to contract law, sustainable development concerns, and issues of war and peace.
The History of International Law collection on HeinOnline includes more than 800 full text publications dating back to 1690 on International Law subjects such as War & Peace, the Nuremberg Trials, Law of the Sea, International Arbitration, Hague Conferences and Conventions.
Oxford University Press has compiled a useful reading list of open access book chapters on the History and Theory of International Law.
Rights and Civilizations: A History and Philosophy of International Law by 
The Hidden History of International Law in the Americas: empire and legal networks by 
System, Order, and International Law: The Early History of International Legal Thought from Machiavelli to Hegel by 
International Law and Empire: historical explorations by 
The Oxford Handbook of the History of International Law by 
Research Handbook on the Theory and History of International Law by 