The crime of aggression targets the individual who is in ultimate control of the political and military action of a state.
The prosecution of the crime of aggression allegedly committed by Russia against Ukraine should be maximised to create a deterrent effect on other countries that might be "tempted" to engage in similar behaviour, says Beth Van Schaack, the US Ambassador-at-large for global criminal justice.
"I have seen an incredible evolution in terms of the world being increasingly united about the imperative of justice, not only to vindicate those victims and survivors whose life plans have been indelibly interrupted by Russia's terrible war of aggression but also to create a deterrent effect," Van Schaack told Euronews in an interview.
"Other states that might be tempted to engage in wars of aggression in their own neighbourhoods would think twice because they would see a robust justice response for the crime of aggression and also for the war crimes and crimes against humanity that might flow from committing the initial act of aggression."
The prosecution of the crime of aggression has been high on the agenda ever since the Kremlin launched the unprovoked invasion of Ukraine, a decision the vast majority of the international community has deplored as an egregious violation of the United Nations' Charter and Ukraine's sovereignty.
A UN resolution endorsed in February by 141 countries denounced "the dire human rights and humanitarian consequences of the aggression by the Russian Federation against Ukraine" and called for the immediate cessation of hostilities and the unconditional withdrawal of all Russian troops.
But despite the growing chorus of voices pleading for accountability, the prosecution of crimes of aggression remains a formidable legal challenge with no clear path forward.
The last time this kind of crime was brought to justice was during the Nuremberg trials held after World War II when the charges were known as "crimes against peace."
"Conversations are happening. Negotiations are still underway," Van Schaack, who coordinates America's legal response to atrocities committed around the world, said in the interview.
"A number of different states are very committed to ensuring meaningful accountability for the crime of aggression, including the United States. And so we're looking for modalities and ways to do this."
Unlike war crimes, crimes against humanity and genocide, which are applied to the individuals who personally commit the atrocities, such as military officers and mercenaries, the crime of aggression is a leadership crime that targets the person ultimately in charge of controlling the aggressor state.
The aggression in itself can consist of an invasion, an occupation, an annexation, a blockade of ports, a bombardment or any other assault that involves the use of weapons by a state against another.
According to the International Criminal Court (ICC), the crime of aggression relates to "the planning, preparation, initiation or execution, by a person in a position effectively to exercise control over or to direct the political or military action of a State, of an act of aggression which, by its character, gravity and scale, constitutes a manifest violation of the Charter of the United Nations."
This makes President Vladimir Putin the likeliest defendant in a future trial.
That possibility, however, remains an abstract aspiration at best.
Heads of state enjoy immunity from prosecution under international law and a trial in absentia could be seen as illegitimate in the eyes of many.
Although the ICC established jurisdiction over crimes of aggression under the so-called Kampala Amendments, this only applies to countries and nationals from countries that are party to the Rome Statute, something that neither Russia nor Ukraine is.
The ICC can also gain jurisdiction by referral of the UN Security Council, an alternative avenue that is all but guaranteed to be blocked by Russia, a permanent member of the council, and also possibly China, one of Moscow's closest allies.
As a possible breakthrough, the European Union has floated two legal options: an ad-hoc tribunal based on a multinational treaty or a hybrid tribunal based on a country's justice system but with elements of international law.
That country would in all likelihood be Ukraine, whose Criminal Code explicitly penalises the "planning, preparation and waging of an aggressive war" with prison sentences of up to 15 years.
While discussions between policymakers and legal scholars unfold, Western allies have agreed to set up the International Centre for the Prosecution of Crimes of Aggression (ICPA) to collect and analyse evidence for a future trial centred on the crime of aggression.
Ambassador Van Schaack welcomed the ICPA as an "important" interim step that can help lay the groundwork for a strong legal case.
"Eventually, there will be a desire and an interest in potentially confirming charges against particular individuals. And that is when we will need a tribunal," Van Schaack said.
"I imagine that negotiations will continue throughout the spring and into the summer. And then ideally, something would be established toward the end of this year," she added.
"States are very committed to coming up with the appropriate model in order to maximise the ability to prosecute the crime of aggression successfully with maximum international involvement and legitimacy."
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