The bombing of a train station in Ukraine where many were gathered to evacuate. The murder of countless civilians in Bucha and other areas. As evidence of Russian atrocities against Ukraine builds, so do calls to bring the perpetrators to justice — including from US President Joe Biden, who recently said Vladimir Putin should be tried for war crimes.

“You saw what happened in Bucha,” Biden told reporters on Monday. “We have to gather the information. And we have to get all the detail so this can be an actual, have a war crimes trial,” Biden said, calling Putin “a war criminal.”

While it’s possible to try war crimes in national courts, investigators from the International Criminal Court (ICC) are already working in Ukraine to gather and vet evidence, and a number of nations have already referred the case to the global court, signaling a strong push to to bring such crimes to trial.

But it’s not as simple as filing a case at a courthouse; there are practical and political limits to what the ICC can do in any of the crimes it investigates and prosecutes. Among those challenges, in this case, is the fact that neither Russia nor Ukraine is party to the ICC, although Ukraine recognizes the court’s jurisdiction, so the court can prosecute those responsible for atrocity crimes committed in Ukraine.

The ICC itself is based in The Hague, the Netherlands, but it has 123 member nations all over the world. The court’s remit is to try grievous crimes like war crimes, genocide, crimes against humanity — collectively known as atrocity crimes — and aggression, but it’s not intended to replace national courts, explained Kelebogile Zvobgo, assistant professor of government at the College of William & Mary. “It’s a court of last resort,” she told Vox. “The court only has jurisdiction in places unwilling or unable to investigate or prosecute their own cases.” Given the fact that the Russian government denies waging war in Ukraine in the first place, much less committing atrocities there, the ICC could be an appropriate mechanism for holding Kremlin officials accountable. But the ICC is not the only avenue to pursue justice for atrocity crimes, and it’s far from guaranteed that Putin or any of his high-level associates would ever stand trial.

A permanent international court is still relatively new

Although the idea for a permanent international criminal court dates back to 1870, the ICC wasn’t established until 1998. The Rome Statute — the product of the UN’s Rome Conference, where 160 different governments convened to consider an international criminal court — enshrined the ICC as the first permanent international court; it came into force in 2002, after 60 countries had ratified the Rome Statute. The ICC has a permanent, professional, and impartial staff, and operates in coordination with the United Nations, although it’s an independent body.

Prior to the court’s establishment, there were mechanisms for trying crimes of international concern, most notably the Tokyo and Nuremberg tribunals after World War II. These were conducted before the Geneva Conventions were passed and were the first known international trials for crimes conducted during conflict. But those trials weren’t immune to criticism, including about their expedience as well as concerns over a sense of partiality, or “victors’ justice,” as Zvobgo said.

Later tribunals, like the United Nations International Criminal Tribunal for the Former Yugoslavia prosecuting the ethnic cleansing of Albanians in Kosovo under former Serbian President Slobodan Milošević, the Special Court for Sierra Leone, which prosecuted those responsible for that nation’s brutal civil war, and the Extraordinary Chambers in the Courts of Cambodia, which prosecuted the crimes of the Khmer Rouge, operated in conjunction with or under the auspices of the UN.

Individual countries can also try individuals for crimes that fall under universal jurisdiction, like atrocity crimes. Most recently, German courts were able to secure convictions for two Syrian military leaders for crimes committed against Syrians in Syria — crimes which technically didn’t involve Germany at all, but because they were so egregious and such an affront to the international order, they fall under universal jurisdiction.

Unlike other international courts, like the European Court of Human Rights, the ICC can only try individuals, not nation-states. That theoretically includes sitting heads of state, although that’s never happened in the court’s 20-year history, and is unlikely to happen in the context of Russia’s invasion of Ukraine. The court has no enforcement mechanism, so while it can issue arrest warrants, it relies on national authorities to execute those warrants. “There are many ICC fugitives,” Zvobgo said, including former Sudanese dictator Omar al-Bashir, who even evaded capture in South Africa, a signatory to the Rome Statute, in 2015. All told, defendants in 11 ICC cases remain at large.

The court has, however, seen 30 cases, with 10 convictions and four acquittals. That might not seem like much, but considering how difficult it is to build the kinds of cases the ICC prosecutes, and the capacity that many defendants have to evade capture and trial, it’s significant. It’s also a sign that countries are following up on their responsibilities, per the Rome Statute, and holding their own investigations and prosecutions for atrocity crimes, Zvobgo told Vox, citing an instance in Colombia in which the ICC closed down a preliminary investigation into grave crimes of international concern — including thousands of alleged extrajudicial killings that occurred over five decades of armed conflict — after determining that the Colombian government could conduct its own investigation and trials.

Prosecuting Putin could be impossible

The ICC doesn’t try defendants in absentia, or if they’re not present at the court. And because the court doesn’t have a mechanism like a police force to enforce its arrest warrants, Putin could evade capture as long as he stays in Russia or other friendly nations — and in power.

“I don’t really see the mechanism for holding Putin criminally accountable,” Zvobgo told Vox. “The US and allies, I don’t think it’s possible that they will seize Putin,” she said, noting that it could set a disastrous precedent and could enable Russia or any other country to use international justice to retaliate against their adversaries.

Plus, there is little precedent for trying sitting heads of state. The only time that’s happened is when Milošević stood trial and was indicted for atrocity crimes in Kosovo in 1999 in a special tribunal convened by the UN. The ICC and other international tribunals have indicted former heads of state, like former President of Liberia Charles Taylor and former President of Chad Hissène Habré, however.

Another complicating factor is that one of the most vocal nations suggesting Putin to be tried at the Hague — the United States — isn’t itself a party to the ICC. The US government voted against the ICC during the Rome Conference in 1998; former President Bill Clinton signed on to the Rome Statute in 2000, but never submitted it to Congress for ratification. Former President George W. Bush in 2002 notified then-UN Secretary-General Kofi Annan that the US would not ratify the Rome Statute and didn’t have to abide by any of its provisions.

“It really shows a lot of hypocrisy,” and encourages the perception of “justice for thee, not for me,” Zvobgo noted. In 2020, the US was under investigation by the ICC for war crimes in Afghanistan, which prompted former President Donald Trump to pursue sanctions against then-ICC prosecutor Fatou Bensouda of The Gambia and senior prosecution official Phakiso Mochochoko, a diplomat from Lesotho.

Even if it were possible to bring Putin to the Hague, the ICC couldn’t try him for one of the most critical crimes — aggression — for which he’s clearly responsible. That’s because the ICC can only try aggression crimes, defined as “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 . . . constitutes a manifest violation of the Charter of the United Nations,” per the Rome Statute, if the countries in question are signatories. Neither Russia nor Ukraine is. Linking Putin to other reported war crimes in Ukraine, like the indiscriminate killing of noncombatants, targeting civilian facilities like train stations and hospitals, and sexual violence, is a massive undertaking and requires documentary evidence — like specific orders or testimony from insider witnesses, which are closely guarded — linking the actions of soldiers on the ground to officials in the Kremlin. “This stuff just takes a long time,” Zvobgo told Vox, “and it doesn’t necessarily end in a guilty verdict.”