At the risk of starting a flame war, there is no international "law" as some people consider it. There are numerous signed agreements, such as the Geneva Conventions, international agreements on maritime rules and related economic zone rules and so forth. There are also precedents, such as the various precedents that came from the Nuremberg trials. Then there are the more general Augustinian concepts of jus ad bellum and jus in bello. The Universal Declaration of Human Rights is a wonderful document to be sure, but it is an aspirational document and nothing more.
There is very little of what most people consider international law that is accepted by all countries, and several parts of this "law" that some countries specifically reject. The most important thing is that there is no enforcement mechanism, no police car that will pull up with flashing lights and "international law police" written on the side. While selling and receiving stolen property may be illegal, if property stolen in town A is openly sold in town B and the town B government and police do nothing about, what force does that law have. If the USA and the EU treat Myanmar as a pariah because of what they are doing to the Rohingya but China continues to trade with them, advance credits etc what next - is anyone "arresting" China as an accomplice?
International rules of behavior; commercial, peacetime, and wartime, and aspirational documents like the Universal Declaration of Human Rights are good things, except as they are signed treaties and have some sort of enforcement mechanism, they are not "law". The reality is all of these often end up as guidelines for "victor's justice" - if you will statutes on how to prosecute violations.