The Si Omnes clause was a clause in the Hague Convention that was signed around the end of 1890s. Let me give you a little background; eversince Sir Henry Dunant went back from witnessing the Battle of Solverino (18-something i cant remember but its in Italy n_n), he saw how massively destructible wars can be upon humanity.. He concluded that the world needed to address this concern, and he produced 2 claims; the world needs a body that ensures and protect humanity in war, and that the world needs a written rule that protects humanity in war..
His first claim is the background of the establishment of the ICRC --International Committee of Red Cross-- and his second claim was reflected in the Hague Convention --later replaced by the Geneva Conventions--. Hague Convention exploded the fathom of humanity in war, that even in conflict zones that should regulate protection (for civillians --women children specifically--, etc), however the trumping factor of Hague convention was that it applied the Si Omnes clause --a clause that states that the treaty only applies for parties that ratified the treaty. I dont know about you, but this is terrifying. Imagine a conflict where one of the parties ain't party to the Hague convention, that literally translates that in that war, anything and everything is both permissible and excusable, including torture, victimizing civillians, taking hostage, etc etc.. I think this is partly due to the fact that maybe at the time the idea of humanity in war hasn't evolved yet as a customary principle (international law's main source is customary values --values that all nations believe in--, meaning that the jus cogen nature of that value itself is the bargaining power for a regulation to actually become enforcable internationally), so that it cannot force binding power to states to adhere to the convention.
I personally thank God that the world came to its senses, starting in 1929 with the First Geneva Convention, which then in 1949 evolved even visual; Protection of the wound, sick, shipwrecked --for soldiers, civilians and Prisoners of War (PoW)--. The world sees Geneva Convention trumping the Si Omnes of the Hague, as now under the current convention, every nation is obliged to adhere to the rules of engagement of war even if the enemy is not party to the convention. So human rights are even more protected.
I agree if some is still testing this convention. But i say thats whats profound about International Law, the level of vagueness and lack of enforcable power gives room for Oppenheim and Austin to debate a debacle that runs even till this day.. is International Law really a law?
My opinion is that at least we have a better starting point nowadays; at least the convention of humanity in war actually is heading the right direction this time. Imagine if US did what it did in Afghanistan, where the Taliban is not party to the Geneve Convention? What would happen? Gross violation of human rights.. in short; a fiasco!
This is why i wrote this, because i'm thankful that Si Omnes is no more..