The invaluable and immense three-volume study on customary international humanitarian law conducted by the International Committee of the Red Cross (ICRC) and published by Cambridge University Press in 2005 is now available free online: https://www.icrc.org/customary-ihl/eng/docs/home. I have used this work to find, for example, relevant sections of a foreign country's military manual.
The Study has two parts:
- Rules - a comprehensive (but not exhaustive) analysis of the customary rules of international humanitarian law identified by the Study and considered to be applicable in all armed conflicts.
- Practice - for each aspect of international humanitarian law covered, a summary of relevant state practice including military manuals, legislation, case law, and official statements; practice of international organizations, conferences, and judicial and quasi-judicial bodies.
We have this work in print here: http://morris.law.yale.edu/record=b591517~S1.
A complementary work, Perspectives on the ICRC Study on Customary International Humanitarian Law, is here: http://morris.law.yale.edu/record=b668444~S1.
"Perspectives on the ICRC Study on Customary International Humanitarian Law results from a year-long examination of the Study by a group of military lawyers, academics and practitioners, all with experience in international humanitarian law. The book discusses the Study, its methodology and its rules and provides a critical analysis of them. It adds its own contribution to scholarship on the interpretation and application of international humanitarian law."
The ICRC has two other databases:
- Treaties - contains treaties, commentaries, and other documents related to international humanitarian law: https://ihl-databases.icrc.org/ihl.
- National Implementation - provides documentation and commentaries concerning the implementation of international humanitarian law at the national level: https://ihl-databases.icrc.org/ihl-nat.