Like many recent viral images from the US-Mexico border, the Reuters image widely circulating on Twitter today is both hard to look at and hard to look away from. In the foreground, a woman – the mother, we assume – has gripped two small children hard by their arms and is dragging them away from a tear gas canister that is smoking just behind them. Behind that, a barbed-wire fence marks the US border.

The children are young, young enough to be wearing nappies, and one of them looks poised to fall over in the sand, unable to keep up with her mother’s urgent pace, perhaps taken aback by the sudden violence with which she is pulling them away from some unknown danger. The mother’s expression is grim. On her t-shirt, there’s an image of Elsa, from the Disney cartoon Frozen. What hopes and dreams did America once hold for her? Or has all her energy been focused on protecting her children from immediate danger? We do not know. Though we can assume that, of all the things that might have kept this mother awake at night, she would not have imagined that US troops would teargas her toddlers.

According to The Washington Post, around 7,400 members of the so-called caravan of migrants from Central America have now reached the Mexican border cities of Tijuana and Mexicali. Most of the caravan’s members, who travelled in a large group for safety, have fled extreme violence and poverty in Honduras.

Having finally reached the border, their progress has halted. The US has said that it can only process up to 100 asylum applications a day, though it reportedly often processes far fewer, leaving thousands of migrants and asylum seekers facing the prospect of spending weeks or even months in crime-ridden Tijuana, the fifth most violent city in the world. Many of them are seeking shelter in an overcrowded, squalid sports centre outside the city.

The situation escalated on Sunday when a group of migrants and asylum seekers attempted to cross the border anyway. US customs and border officials shut the official border at San Ysidro, the busiest port of entry between the US and Mexico, for several hours and fired tear gas to disperse the crowds. The head of US Customs and Border Patrol, Carla Provost, told Fox and Friends that agents had fired the gas after some of the crowd threw bottles and rocks at them.

The images of toddlers fleeing tear gas represents another stomach-churning symbol for how, under President Donald Trump, America views its responsibilities towards those who seek safety from violence and persecution.

As the New Yorker points out, now that the Trump administration is seeking to cut a deal with Mexico requiring asylum seekers to wait there while their claim is being processed, the horrifying new developments at the Mexican border could become a new normal. That is a chilling prospect.

In response to Sunday’s developments, Trump tweeted that “Mexico should move the flag waving Migrants, many of whom are stone cold criminals, back to their countries. Do it by plane, do it by bus, do it anyway you want, but they are NOT coming into the U.S.A. We will close the Border permanently if need be. Congress, fund the WALL!”

To be very clear, what the president is suggesting will almost certainly cause innocent people to be killed. For some of those in the caravan, returning home means almost certain death. The president’s suggestions are not only monstrously inhumane, they are also illegal under both international law and America’s own domestic legal provisions.

According to the 1951 Refugee Convention and its 1967 protocol, which the US signed up to in 1968, the US has a legal obligation to provide protection to anyone who enters the country and presents a “well-founded fear of being persecuted because of his or her race, religion, nationality, membership of a particular social group or political opinion.”

As this Amnesty International report points out, threatening to return asylum seekers to their home country or closing the border to asylum seekers is in clear contravention of international law:

Under international law, non-admission of asylum-seekers at the border directly violates the principle of non-refoulement, which is binding on the US as a party to the 1967 Protocol of the 1951 UN Convention Relating to the Status of Refugees and as a principle of international customary law. The non-refoulement principle, provided for in Article 33 of the Convention, provides that states must not return persons to territories where their “life or freedom” would be threatened. This principle is the cornerstone of all international law on refugees and considered completely non-derogable by states. An advisory opinion interpreting the Convention from the UNHCR60 holds that this principle is violated in situations of non-admission at the border and applies to return not only to countries of origin but to “any other place” where a person has reason to fear for their life.

Under the Trump administration – and indeed under previous presidencies too – border officials have broken this law by refusing asylum seekers entry at official ports. Numerous media reports (including my own reporting) at the height of the family separation crisis earlier this year demonstrated that a significant number of families arrested for entering the US at an unofficial border had initially tried to cross at an official border crossing and been turned away.

If the Trump administration continues to threaten to close US borders to those seeking asylum, many desperate individuals will feel forced to use more dangerous, unofficial crossings. According to Amnesty, the number of migrant deaths recorded in Pima County in the Arizona desert between November 2016 and April 2017 doubled on the same period a year earlier, suggesting that already migrants are taking more risky routes in the hope of finding safety in the US. The president will have still more blood on his hands.

For Americans, the latest images emanating from the US-Mexico border should be a source of national shame in a country that once prided itself on its moral leadership, on its open welcome of those seeking freedom and opportunity. But they should also worry observers further afield, as anti-immigrant, populist parties gain traction in Europe and beyond. The Refugee Convention was drawn up in the horrifying aftermath of the World War II and was intended to offer a moral and legal blueprint for how to treat the victims of conflict. We cannot afford to go back. The children fleeing tear gas at the US border cannot afford to go back either.