The Russian invasion of Ukraine should finally end the administration’s fantasy that Moscow will help stop the war in Syria.

And it ought to force the White House to forge a new strategy to deal with the most shocking humanitarian crisis of the century, which is spilling over from Syria to all of its neighbors. Otherwise, the level of human suffering will get much, much worse.

U.S. officials have insisted for three years that there was no military solution in Syria; they clung to delusions that Russia would convince Bashar al-Assad to make way for a transitional government and free elections. But those hopes reached a dead end at failed Geneva peace talks, where the Russians refused to pressure the Syrian regime.

Just as Vladimir Putin used military force in Ukraine to try to restore an ally to power, the Russian leader has armed and encouraged Assad to retain power at any human cost.

Assad’s key weapon is his willingness to commit the most brutal war crimes. That means besieging whole towns, starving residents, targeting civilians with mass bombing of residential areas, and poison gas. His planes have deliberately destroyed schools and hospitals; his militias try to kill doctors in rebel-held areas.

Meanwhile, Assad insists that all humanitarian aid be funneled through the Syrian government; United Nations agencies and many private aid groups are wary of disobeying lest he shut down their operations in Damascus. As a result, much of the aid never reaches the neediest civilians.

Assad’s goal is to depopulate cities and towns held by rebels and drive their populations into neighboring countries, or displace them within Syria. The regime hopes this strategy will force the opposition to quit, and will pressure Arab neighbors to end support for the rebels. When it comes to war crimes, there are no holds barred.

“This is a war without end, a war without limits, a war without law,” said David Miliband, the former British foreign minister who now heads the International Rescue Committee, one of the most active humanitarian agencies helping Syrians in need. “The targeting of civilians has happened before, but the way it is happening here, by barrel bombs [filled with shrapnel and dropped on residential areas], by starvation . . . people thought this wouldn’t happen again.”

Nearly half the Syrian population, at least 9.3 million people, are either refugees abroad or in desperate straits within their own country. Whole cities — such as glorious, historic and ancient Aleppo, have been largely reduced to rubble by regime bombers. More than 130,000 Syrians have died.

A whole generation of children has become victim, as detailed in a poignant report just issued by Save the Children. It estimates that at least 10,000 children have died in the conflict, with millions more bereft of schooling and health care. In what was once a middle-class country, the medical system has collapsed, leading to an increase in the deaths of newborns and a polio epidemic.

But Bashar bombs on, driving at least 2.3 million Syrians (and perhaps twice that many) to quit Syria. This has created dangerous financial and social tensions in neighboring countries, including Iraq, Turkey, Jordan, and Lebanon. In the latter, with its population of just over 4 million, at least one million Syrian refugees have sought shelter. It’s as if the entire population of Germany had washed up on U.S. shores as refugees.

While the United States has taken the lead in Syrian humanitarian aid, with $1.7 billion, money alone can’t address this huge crisis. On Friday, a bipartisan group of 19 senators, including Robert Casey, D-Pa., called for President Obama to submit a “new humanitarian strategy.” I asked him what he had in mind.

“It’s important that there be a more aggressive approach,” Casey said, because “there are still problems with aid delivery.” That would mean giving teeth to a recent U.N. resolution that calls on Assad to lift sieges of populated areas and allow unhindered aid delivery across borders — without going through Damascus.

Which leads to Casey’s second, and key point: “Given the scale of the suffering, you have to change the dynamic on the battlefield … to arrive at a strategy of getting humanitarian aid delivered and alleviating some of the suffering.”

Specifically, Casey suggested it was time to ensure that well-vetted opposition groups get the weapons they need to prevent government bombing and shelling of civilians, and to guarantee that aid reaches the needy. It may be more difficult to do this today — with a more fragmented opposition — than it would have been one or two years ago. But it must be tried.

I agree. Without such aid, Assad will keep bombing Syrian civilians, backed by Moscow and Tehran. He will depopulate much of his country and put an unbearable strain on his Arab neighbors. The administration needs a new humanitarian strategy for Syria before he succeeds.