Le droit des gens (French)  or Volkerrecht (German)

Going back into history and referring for example to the times of the Roman Empire, the law that was applicable, was between the Citizens of Rome (Roman Republic Law) and the Law regarding the other tribes or Kingdoms of the World (Jus Gentium).

Jus Gentium was also applicable as a section of Common Law to the Citizens of Rome because it´s derivation was at least partly "human " comparatively to "Jus Civile" which was applied to a specific group of people.

"Le droit des gens" (French) or the Law of the People (English) or Volkerrecht (German) is  what is still referred today as International Law, and the heritage or origins have a close relationship to the laws of the Roman Empire.

Modern Scenario : The Russian Invasion of Ukraine 

The International Law Association (ILA) has termed the Russian Invasion of Ukraine illegal or a violation of International Law. The statements regarding the illegality of the invasion, were released on the 25. February. 2022 through the German Division of the ILA.

But does the Russian invasion of Ukraine automatically criminalizes Russia as a Sovereign State and a permanent member of the Security Council of the United Nations? and given that the immunity of States from the jurisdiction or prosecution of other States is a customary rule of international law? State Practices comparatively to the Russian invasion have been expressed severally with regard to the "Opinio Juris" of States.

Compare the commentaries of the International Court of Justice (ICJ) in the Case of "Germany v Italy":
"That practice shows that, whether in claiming immunity for themselves or according it to others, States generally proceed on the basis that there is a right to immunity under international law, together with a corresponding obligation on the part of other States to respect and give effect to that immunity" (ICJ, Jurisdictional immunities of the State (Germany v. Italy: Greece intervening), ICJ Reports, 3 Feb. 2012, para. 56, p. 123).

Should Vladimir Putin be held accountable for the invasion or for crimes committed during the invasion of Ukraine? Maybe or arguably yes, and arguably no.  

In the Arrest Warrant case (Democratic Republic of Congo-DRC v Belgium), the ICJ,

affirmed that, Belgium had breached the total immunity from criminal proceedings enjoyed by Foreign Affairs Ministers, Heads of Governments, and Heads of States, with regard to the international arrest warrant issued by Belgium against the Ex-Minister of Foreign Affairs of the DRC, for possible war crimes committed while in office.

Compare the commentaries of the ICJ in the arrest warrant case:

“1.  (…) certain holders of high-ranking office, such as the Head of State, Head of Government,

"2.  and Ministers for Foreign Affairs (…)”  See paragraph 51, Judgement of 2002

the ICJ further stated that “any act of authority of another State which would hinder [the high official] in the performance of his or her duties” would be considered as a violation of customary international law. Immunity "Ratione Personae" plus immunity "Ratione Materiae" enjoyed by the given high ranking official (say Putin) is total or absolute and serves as a shield against any form of prosecution, even if the crime committed relates to highest breaches of international criminal law.

We as researchers in the interest of mankind are just trying find and present a common ground solution in the interest of all the parties involved with the context of customary international law. 

We are experts and members of the International Law division of the American Bar Association and the International Law Association