The UN Charter, which in 1945 laid the foundations for the UN's missions, powers, rules of procedure and organisation, prohibits military force between states. However, there are exceptions, such as when the UN Security Council authorises the use of force or in the case of self-defence.

"Self-defence means that Ukraine has the right to defend itself militarily against Russian aggression. Russia, on the other hand, had no legal right to enter Ukraine," says Britta Sjöstedt, senior lecturer at the Faculty of Law and a researcher on the environment and armed conflict.

Russia's arguments about the demilitarisation and denazification of Ukraine and the prevention of genocide are immediately dismissed. Russia also points to other situations where unlawful military force has been used. For example, the American invasion of Iraq in 2003 or NATO's bombing of the then Yugoslavia in 1999, neither of which can be classified as self-defence, nor were they authorised by the UN Security Council.

"Pointing to another illegal use of military force does not justify Russia's invasion of Ukraine. Russia's invasion is as illegal as the invasion of Iraq. Even if the circumstances are different, I think it is important to underline that international law does not differentiate according to which country violates it, even if the reactions of the world may be different," says Britta Sjöstedt.

Difficult to bring Russia to justice

One problem is that trials after wars tend to be fair according to the victors' definition of justice. After the Second World War, the victorious powers tried the losers, and no one from the victorious side was held accountable," says Britta Sjöstedt. "The establishment of the International Criminal Court (ICC) in The Hague and international war crimes tribunals is an attempt to make the process fair".

Since its establishment in 2002, the ICC has been able to charge individuals with war crimes, crimes against humanity and genocide. So far, however, only individuals from the African continent have been indicted by the ICC. In certain cases, the ICC can also try crimes of aggression, in other words the decision to invade another country.

A dilemma arises from the fact that neither Ukraine nor Russia has ratified the Rome Statute which established the ICC. Nor has the US.

"However, Ukraine has recognised the court's jurisdiction to a certain extent, meaning that the court can try certain cases in relation to the Ukrainian war. The ICC can indict people suspected of war crimes, genocide and crimes against humanity, as long as the crimes took place on Ukrainian territory after 21 November 2013. However, the court cannot try the crime of aggression because neither Ukraine nor Russia approved the subsequent agreement that added aggression to the crimes that the ICC can try," says Britta Sjöstedt.

The reason the crime of aggression was dealt with separately was that states had difficulty agreeing on a definition. Agreement on the crime of aggression remains difficult, and only about 40 states have agreed to the additional treaty. 

Theoretically, the ICC could issue an arrest warrant for Putin as the person ultimately responsible for the Russian army and other senior leaders in Russia for their systematic war crimes in Ukraine. The Russian leadership would then be unable to travel to the ICC's membership - 123 of the world's nations.

"However, as Russia is not a member of the ICC, he would be safe as long as he remains in the country. No state has the right to exercise jurisdiction by, for example, arresting a person in another state without that state's permission," says Britta Sjöstedt.

In the 1990s, war crimes tribunals were established after the war in the former Yugoslavia in 1991-1995 and the genocide in Rwanda in 1994-1995. Such tribunals are established by the Security Council. As Russia is a permanent member and has veto power, it is unlikely that such a war crimes tribunal could become a reality.

However, there are other plans for a tribunal. The European Parliament wants to establish one to investigate alleged war crimes in Ukraine, as well as the crime of aggression - the invasion.

“It is not clear how this would work. An EU court does not solve the problem of jurisdiction either, in other words the opportunity to bring the accused, Russia and Putin for instance, to trial difficult,” says Britta Sjöstedt.

Putin has immunity

War criminals can also be tried in national courts. The states of the world have an international obligation to prosecute war crimes, genocide, and crimes against humanity, regardless of where the crimes were committed.

For example, in 2022 an Iranian citizen was sentenced to life imprisonment by a Swedish court for serious violations of international law and murder committed in Iran in the 1980s. The case is now before the Court of Appeal. However, it would not be possible to prosecute Putin in Sweden. As head of state, he has immunity in a foreign domestic court, and the only party that can waive this immunity is the country concerned, in this case Russia. This would require a clear change of power in the country. Foreign ministers and diplomats also have a similar personal immunity under international law.

"Immunity applies to heads of state and government for acts performed in the exercise of their functions, even after they have left office." The reason for this is that the leader's actions are so closely linked to the state and as such should be dealt with at an intergovernmental level or in an international court. This is in the process of changing making acts that qualify as international crimes, such as war crimes and genocide, possible for national courts to try after a head of a state is no longer in power, but not all states agree to such a change of the law. On the other hand, the actions of soldiers, such as torture or individual attacks on civilians, can be tried in national courts," says Britta Sjöstedt.

Laws of armed conflict apply regardless

Once an armed conflict - a war - has begun, the laws of armed conflict come into play. Put simply, the laws of armed conflict are those parts of international law that govern the conduct of warfare and the actions of states during war and occupation.

The laws of armed conflict do not consider which country has started the conflict, so they apply to Ukraine and Russia alike. Nevertheless, Amnesty International found itself at the center of a storm of criticism when it accused Ukraine of war crimes in 2022.

"Important principles such as the principle of distinction, which means distinguishing between civilian and military targets; the principle of proportionality, which means that the risk to the civilian population must be in proportion to the expected military advantage of an attack; and the principle of precaution, which means avoiding civilian casualties, also apply to the invaded country. The idea is to balance the possible damage caused by an attack against the military necessity of the attack," says Alberto Rinaldi, a researcher in humanitarian law and human rights at the Faculty of Law.

The fact is that both Russia and Ukraine have violated the laws of armed conflict, albeit to very different degrees, and all of these breaches are not considered as war crimes.

"Ukraine has used schools as shelters, mixing civilian and military use, which is not allowed. But Russia has been accused of much more serious crimes than Ukraine. There are things like mass graves that have been discovered in the course of the war, particularly in Russian-controlled areas. The bodies show signs of torture," says Britta Sjöstedt.

In the spring of 2002, the ICC sent a group to Ukraine to collect evidence and testimony. The group consisted of 42 people, the largest the ICC has ever sent to a war zone.

Gathering evidence is different from the prosecutor's role of bringing suspected criminals to The Hague, but of course evidence is the basis for possible prosecutions, says Alberto Rinaldi:

"When gathering evidence, you use material such as pictures and videos, but you also visit sites and record oral testimony. This is how the mass graves in Rwanda and Yugoslavia were discovered. There is also statistical analysis, such as comparing the number of people in an area before and after the war, and how many people are now missing".

Distinguishing civilians from combatants is not easy

The laws of armed conflict mainly protect civilians. The definition of a combatant is someone who belongs to the armed forces of one of the parties to the conflict or to a group under their command, wears a uniform or other identifying mark and openly carries a weapon. Persons who are not considered to be combatants are civilians and, under the laws of armed conflict, may not be the target of attacks. 

"The distinction between combatants and civilians is central to international humanitarian law. It is also very problematic, especially in cases like Vietnam, decolonisation and the 2001 war on terror, where the lines are not as clear as in a traditional war," says Alberto Rinaldi.

There are also cases where civilians can lose their status. According to Additional Protocol I to the 1949 Geneva Conventions that deals with this issue, civilians can lose their status if they "take a direct part in hostilities". 

"This group could include civilians taking up arms in Ukraine. However, the issue is complex because there is no accepted definition of what constitutes direct participation and when such participation begins or ends," says Alberto Rinaldi.

"For example, what happens if someone drives a truck full of explosives from A to B, but is not responsible for its detonation? Or someone who delivers food and medicine to combatants? On the one hand, the Red Cross is proposing more generous rules, while on the other, countries such as Israel are seeking stricter limits to deal with these civilians," says Alberto Rinaldi.

At the same time, there is a great deal of civilian involvement in Ukraine. For example, there are apps that allow civilians to report the location of Russian soldiers. "The apps are problematic because Ukraine is involving civilians in intelligence activities. It also has the consequence of Russia attacking civilians in order not to reveal the position of its soldiers. But it is probably not enough involvement in the hostilities to say that the civilians should lose their protection," says Britta Sjöstedt.