President Biden has called Vladimir Putin a "war criminal," and said Monday the Russian leader should face a trial over the alleged atrocities in Ukrainian city of Bucha.

Yes, but: While similar calls have echoed worldwide, Putin is unlikely to be held criminally accountable, at least as long as he remains in power.

The big picture: War crimes have been historically hard to investigate and often even more challenging to prosecute.

• This is especially true when prosecutors seek to hold leaders or former leaders accountable.

For clear cases of war crimes, often the main challenges are determining who is responsible, and what evidence exists that can establish culpability, according to Alex Whiting, a Harvard Law School visiting professor and deputy specialist prosecutor at the Kosovo Specialist Prosecutor’s Office in The Hague.

• High-profile leaders often aren't at the scene of alleged war crimes, making them harder to prosecute.
• In Bucha, for example, where reports have emerged of a mass grave and bodies of civilians strewn in the city's streets, the main challenge for investigators is determining who is responsible and how high up the chain of command the responsibility goes, explains Whiting, who previously served as a prosecutor at the International Criminal Court.
• For its part, the Kremlin has rejected the reports of atrocities in Bucha, and previously called Biden's "war criminal" remarks "unacceptable."

Even if prosecutors can show that high-level officials and/or Putin directed or were aware of orders or actions that may constitute war crimes, a trial at the ICC or a war crimes tribunal cannot be conducted unless the official is in custody.

• Because Russia is not a member of the ICC, which is conducting an investigation into possible war crimes committed in Ukraine, there would be no expectation that Moscow would hand over Putin or any other official if they're charged.
• Many individuals who have gone to trial for war crimes, were either captured during the armed conflict or were handed over after they fell from power — an unlikely scenario for Putin.

The bottom line: Even though it's unlikely Putin will face punishment (outside of sanctions and global condemnation) for any war crimes committed in Ukraine, investigating and possibly bringing forward charges is important, Whiting says.

• It "sends a message to the victims that they're being seen and recognized" and it tells perpetrators that they're being watched."

Go deeper: What counts as a war crime and why they're so hard to prosecute