When President Joe Biden urged world leaders on Tuesday not to diminish support for Ukraine, he used a phrase whose importance you may have missed. Speaking at the U.N. General Assembly, Biden charged (correctly) that Russia’s price for peace is “Ukraine’s capitulation, Ukraine’s territory and Ukraine’s children.”

I’ve added italics to those last two words because of Moscow’s policy of illegally transferring tens of thousands of Ukrainian children to Russia proper, or Russian-occupied parts of Ukraine, and trying to transform them into good little Ukraine-hating Russians.

In his own speech to the United Nations, Ukrainian President Volodymyr Zelenskyy also denounced Russia’s seizure of his country’s children as “purely a genocide.” That Russian war crime goes to the heart of why Ukraine believes it must win this war.

According to official data from the Ukrainian government, at least 19,546 children have been transferred illegally to Russian control since the war began. However, those numbers only include cases reported by a parent or guardian. The real figures are probably much higher, and there is no record of whether those children have been adopted or sent to Russian orphanages. Only 386 have been returned.

In a war where bombing civilians is central to Russian military strategy, no war crime seems too heinous to Moscow — from bombing schools, markets and hospitals to torture, rape and murder in Russian-occupied cities.

Yet there is something especially evil about kidnapping children, which relates directly to Vladimir Putin’s belief that there is no such thing as a Ukrainian nationality and that the state has no right to exist.

According to this thinking, deporting Ukrainian children makes sense: All Ukrainian youngsters should ultimately be “reeducated” to love the Russian motherland and despise the Ukrainian “Nazis.”

“Forced deportation and adoption of Ukrainian children is one of the elements of a war of genocide,” Ukrainian Prosecutor General Andriy Kostin told me when I visited Kyiv in July. “This is a matter of intentional policy.”

That is why the International Criminal Court has issued arrest warrants for Putin and his commissioner for children’s rights, Maria Lvova-Belova, alleging their responsibility for the war crime of unlawful deportation of children from occupied areas of Ukraine.

Russia has weaponized food (blocking Ukrainian grain exports) and energy (making threats to the Zaporizhzhia Nuclear Power Plant, which it occupies), as Zelenskyy pointed out. It has also weaponized ecology, blowing up the Nova Kakhovka dam, which flooded cities, farmlands and animal life. As if this was insufficient, Putin has even weaponized Ukrainian children.

“The Russians use different ways to take the children out of Ukraine,” Kostin related, “not only taking them from orphanages but from families.”

In some cases, parents in occupied Ukraine were encouraged to send their children for “rest and recuperation” to camps in Crimea that sought to reeducate them. Many children were bused there directly from school without parental permission. Many never returned.

During the Russian destruction and occupation of Mariupol and other cities, many thousands of children from orphanages, boarding schools, shelters and hospitals were taken to Russian-controlled areas, even though they might have had family members elsewhere in Ukraine. Children weren’t allowed to call friends or relatives, who had no way to find them. Phones were taken away. Young children were put up for adoption.

Often, children were in shelters because their families had been killed by Russian bombing, as in Mariupol, where Russia flattened the city and refused to permit a humanitarian corridor to let civilians exit.

Moscow has made propaganda out of kidnapped children, filming youngsters being given toys and candy and being “happily” adopted. “Children are a sacred cause. We took them out of the conflict zone, saving their lives and health,” said the utterly cynical Putin in June. Naturally, he never mentions that most of the children were displaced or orphaned by Russian bombs.

A recent documentary called “Uprooted,” produced by the Kyiv Independent newspaper and available on YouTube, interviews some of the few children who were rescued by incredibly brave relatives or guardians.

I interviewed one of those relatives featured in the film, Valentina Yermachkova, who was a 19-year-old student in Dnipro when the war started. Her mother and brother were killed by a Russian shell as they sought food after the Russians invaded Mariupol. Her two younger sisters were moved to a hospital in occupied Donetsk, but not allowed to contact relatives. Fourteen-year-old Sofiia had hidden her brother’s phone and eventually managed to sneak a call to her older sister.

Yermachkova, on her own, took buses through Poland, Lithuania, across Russia and back into occupied Donetsk, and demanded the return of her siblings. “I was afraid, but I knew I had no choice,” she told me when I met her and her sisters in Dnipro. “Now I realize I might have gotten stuck there, too.” She now studies law in the hopes she can work on efforts to rescue other Ukrainian children.

Rescuing children is one main reason why Prosecutor General Kostin was set to arrive in Washington, D.C., this week to discuss the issue with top U.S. officials; it will also be raised at side meetings during the U.N. General Assembly. “Bringing our children back is a priority,” Kostin told me, “for us as a state and for the civilized world.”

“Countries in the global south that suffered in the past from violations of basic human rights need to understand that this evil will return if they put their short-term interests higher than their values,” Kostin insisted. He was referring to countries such as South Africa, which maintains close ties with Russia and isn’t supporting Ukraine.

Whether Ukraine can retrieve its stolen children is a huge question mark, and U.S. officials should offer any help requested. But public focus on the issue is rightly aimed at those leaders worldwide, and in this country, who still seek to shake Putin’s hand.

Trudy Rubin is a columnist and editorial board member for the Philadelphia Inquirer.