The world is witnessing in Syria a humanitarian catastrophe of historic proportions. It is being perpetrated by Russian President Vladimir Putin in support of his protégé, Syrian President Bashar al-Assad. Russian warplanes are bombing the civilian population of Aleppo, the country’s most populous city, to assist Syrian government forces attempting to take control of rebel-held areas.

The combined assault has, among other things, killed hundreds of people and wounded more than a thousand, put the city’s remaining hospitals out of commission, and deprived the population of drinking water.

Putin is moving aggressively to exploit the three months between now and the January 20 inauguration of the next US president, based on the callous calculus that the United States will be mostly immobilized by the political transition. As The New York Times puts it: “Putin calculates that the departing President Obama will be unlikely to intervene in the escalating Syrian conflict and a new American president who might consider a tougher policy will not yet be in office.” The Times then quotes Russian political scientist Nikolai V. Petrov: “The next American president will face a new reality and will be forced to accept it.”

Other reports in the Times and elsewhere have vividly depicted the suffering of Aleppo’s people and the heroic efforts of the doctors and civilians, like the White Helmets, who are risking their lives to help them. When the facts are fully established, Putin’s bombing of Aleppo will be viewed as among the modern world’s most egregious war crimes.

I appeal to the people of Russia, the US, Europe, and the rest of the world not to stand idly by, but to spread the word and voice their outrage. An outpouring of public opinion could induce Putin to call a halt to his heinous crime against humanity.