A act of aggression is a use of force against the territorial integrity or political independence of a state that has not been authorised by the UN Security Council and is not in self-defence.

This act of aggression can be considered an international crime. This means that those who directed the military actions, or contributed to shaping and influencing them, are criminally responsible.

The crime of aggression is proscribed in the statute of the International Criminal Court, as well as in many national legal systems (including the criminal codes of Russia Ukraine).

The ICC does not have jurisdiction over crimes of aggression without a referral from the UN Security Council. Russia, as a member, has veto rights over the UN Security council, which they would likely use. Therefore, the ICC cannot prosecute Russia for the crime of aggression.

Ukraine, in 2014, did make a declaration which allowed the ICC to investigate allegations of war crimes, crimes against humanity and genocide on its territory. 40 further states have referred the situation in Ukraine to the ICC. The ICC has, therefore, opened these investigations.

However, as the ICC cannot investigate Russia for crimes of aggression, there is a gap in the international law. An international tribunal would fill this gap by creating a system in which those most responsible for the crime of aggression would be held accountable. This would be complementary to the investigations of the ICC, and would support its work.

A body dedicated to the crime of aggression would speed up the investigation and prosecution process for these crimes. It would also reinforce the sense of global solidarity, and prevent the potentially serious threats a nation may face if they pursued Russia for this crime on its own. It may also encourage those who fear the investigation or prosecution to part from the current Russian leadership.

The UN can recommend the enforcement of action under the UN Charter. However, it is generally understood that it cannot taking binding action. Therefore, whilst the UN General Assembly may call for the creation of such a tribunal, it cannot create the tribunal itself.

The tribunal would be created to assist the pursuit of peace and advance the international rule of law, and so operates as another international accountability mechanism. This is because:

1. This action may deter possible perpetrators, and dissuade them from further crimes. They may be pushed to end the war quicker in return for immunity or non prosecution.
2. This international mechanism would de-politicise the investigation. Future peace-talks between Ukraine and Russia could be on a basis that the figures in question are not investigated by the tribunal, encouraging the preparators to find a peaceful resolution to the war faster.
3. If these prosecutions were held by individual countries, they may be seen as unfair or political. Issues could also be raised that the country undertaking the prosecutions may not have the capacity or competence to do so. Internationalisation, however, shows the investigations are necessary under international law.

Under international law, a very small number of state officials are entitled to immunity from criminal jurisdiction whilst they are in office. This only applies to current Heads of State, Heads of Government and Foreign Ministers. There may be grounds to contest this immunity when prosecuting for international crimes (although the scope of these grounds are contested). Irrespective of how this important issue is addressed, however, the mere fact that immunity may be claimed by a very limited number of individuals is no reason to stop the creation of an international tribunal.

This proposal is a response to a request by Ukraine, and reflects the response of many countries who offer to seek assistance. This is not an indication that the situation is exceptional, and tribunals could be created on similar grounds for other crimes of aggression in the future.

This is a question for the future. What matters now is the urgent need to demonstrate that the crime of aggression will not be tolerated in relation to Ukraine, and that the perpetrators can be expected to be investigated and prosecuted. In 1942, when the grounds for what would become the Nuremberg trials were created, it seemed unlikely that Nazi war criminals would see justice. In 1946, at the trials, 161 Nazi war criminals were prosecuted and punished.