All bets are off when it comes to regulating war. Such a condition is virtually by definition beyond the confines of law. Even international law is but an impotent dwarf next to the raw force of a governmental regime at war—whether with its own citizens or another country. To be sure, the International Criminal Court had by 2017 made a dent in holding some perpetrators of atrocities such as genocide accountable for their deeds. Such efforts were still the exception, unfortunately, when Russia, China, and Bolivia vetoes a resolution in the U.N. Security Council that would have penalize Syria’s Issad regime for having used chemical weapons on Syrians. The reasons for the vetoes—and the fact that Egypt, Ethiopia, and Kazakhstan all obstained—implies that holding perpetrators accountable by international means had not yet become a priority at the international level.
The full essay is at "China and Russia Protect Syria's Assad."