Rex Tillerson appears before the Senate Committee on Foreign Relations. (Melina Mara/The Washington Post)

Pierre-Richard Prosper was U.S. ambassador-at-large for war crimes issues from 2001 to 2005 and a prosecutor at the U.N. International Criminal Tribunal for Rwanda. John B. Bellinger III served as the legal adviser at the State Department from 2005 to 2009.

Sen. Marco Rubio (R-Fla.) is right to wonder whether Vladimir Putin is a war criminal. And when he put that question to Secretary of State-designate Rex Tillerson this week, Tillerson was right to reserve judgment on the issue. Here’s why.

As a preliminary matter, Russian troops are engaged in armed conflicts in Syria and Ukraine, where individuals engaged in those hostilities have violated the international law of armed conflict. This body of law includes the 1907 Hague Regulations, the 1949 Geneva Conventions, a host of other applicable treaties, and a large body of well-settled customary international law that prohibits, among other things, targeting civilians and the disproportionate use of force. Despite these prohibitions, combatants on all sides of the Syria conflict have committed horrible atrocities, especially in Aleppo, where Syrian government forces backed by Russia have used bunker-busting bombs in civilian areas and targeted non-combatants as they have attempted to flee.

As former U.S. government officials who were once responsible for compliance with and enforcement of international humanitarian law and the law of armed conflict, we condemn in the strongest possible terms these battlefield crimes. We call upon the United Nations, the United States and all civilized nations to investigate and prosecute those responsible, even if that means holding heads of state to account.

Nevertheless, while we can understand why Rubio would ask if the Russian president is a war criminal, there is a multitude of reasons Tillerson is right to exercise caution before jumping to such conclusions before he even enters office.

First, just as we would expect foreign governments, international organizations and human rights groups to investigate and analyze all the relevant facts before declaring any U.S. president or other senior official a war criminal, so too must U.S. officials engage in careful deliberation before attaching such labels to foreign leaders, no matter how much we may detest them or their policies.

Second, the terms “war criminal” and “war crimes” are legal expressions reserved for those individuals who have been found to have committed certain grave breaches of the laws of war, as set forth in various international agreements and court decisions as well as our own War Crimes Act. Advocacy groups are often quick to declare that war crimes have been committed when civilians are killed or protected sites such as hospitals or schools are damaged in armed conflicts, but the terms should not be used or taken lightly, lest we diminish their impact and meaning when discussing those whose acts genuinely warrant the labels. For the same reason, both the George W. Bush and Obama administrations, respectively, conducted lengthy factual and legal investigations before concluding that “genocide” had been committed in Darfur by the government of Sudan and in Iraq and Syria by the Islamic State.

Third, even if we could be certain (and it does appear likely to us) that Russian troops had committed or supported the commission of war crimes in the field, responsibility for war crimes belongs to those individuals who order or carry out such acts with the necessary criminal intent or knowledge. Consistent with due process and fundamental fairness, such determinations must be made on an individualized, case-by-case basis through an impartial and objective analysis of all the factual evidence and the law. Anything less rigorous would violate well-established fundamental rights and would be unjust.

This is not to say that Putin is innocent or that he bears no responsibility for Russia’s misconduct on the battlefield. Far from it. Russian involvement in the targeting of civilians must be investigated, and if Putin ordered or even approved the commission of war crimes, he should be held accountable. However, we think it wise and advisable for Tillerson to learn all the facts — including intelligence information not previously available to him — and to carefully analyze the law through an appropriate process before attaching the powerful label of “war criminal” to someone engaged in armed conflict, whether a foot soldier or a head of state. Putin may not deserve it, but that’s both the American way and required by international law.