The 24 hours before Tuesday’s flooding of the Dnipro River basin were already dramatic, as Ukraine appeared on the cusp of a counteroffensive that many in Kyiv see as their best chance to defeat Russia’s invasion.

US President Joe Biden, just hours earlier, gave a fingers-crossed sign in response to a question on the Ukrainian campaign’s chances.

Russia, meanwhile, claimed without evidence it fended off a large armored attack in the country’s eastern Donbas region. But the Kremlin had to dismiss as fake a putative address to the nation by President Vladimir Putin, in which he allegedly called on Russians to rally against invasion by Ukraine in the wake of strikes across the border in recent days.

Nothing, however, compared to what appeared to be a horrific escalation of the war early on Tuesday, as up to 18 million cubic meters (4.8 billion gallons) of water poured through a massive breach in the Kakhovka dam, swamping communities along the banks of the Dnipro river.

Ukraine accused Russia, which occupies the dam, of blowing it up, while Moscow said shelling by Kyiv’s troops was the cause. Intelligence agencies for Ukraine’s allies, including the US, are still assessing who is responsible, but are leaning toward Russia, a Western official said on Tuesday.

President Volodymyr Zelenskiy, in his nightly video address, called the the dam’s destruction “an environmental bomb of mass destruction.”

The motive for Russia to have blown up the dam also was still being assessed, but appears to have been to make it more for difficult for Ukraine to conduct a river crossing downstream, while at the same time creating a significant humanitarian challenge for Kyiv to deal with, the western official said.

That assessment was contested by a person close to Russia’s Defense Ministry. Russian frontline positions have now been flooded, the person said, while in a week to 10 days the reservoir would empty, the floods gone and parts of the river would settle at lower levels than before. That will return the river to patterns recorded in the pre-Soviet era, when in hot summers some parts of the Dnipro became so shallow it could be forded.

Ukrainian officials were skeptical of any military impact. “Sooner or later this would have happened, and it happened at 2:50 a.m. this morning,” Ihor Zhovkva, the deputy head of Zelenskiy’s office, said in a phone interview. “It will have no impact on military operations, and we will make the counteroffensive.”

Zhovkva called on “European and US allies to clearly say that full security for Ukraine can only happen with fully fledged EU and NATO membership.”

In fact, the impact of the dam breach may be bigger off than on the battlefield, as the unfolding humanitarian and ecological disaster refocuses the attention of Ukraine’s international backers, after more than 15 months of war and tens of billions of dollars spent on the nation’s military and financial support.

“This is something that has a new dimension, but which fits with the way in which Putin wages this war,” German Chancellor Olaf Scholz told WDR television, adding that it underlined the need to support Ukraine.

“Intentionally attacking exclusively civilian infrastructure is a war crime,” UK Foreign Minister James Cleverly said on Twitter.

Ukrainian authorities said about 1,500 people had been evacuated from homes on the western bank of the Dnipro. It wasn’t clear how many people were affected on the Russian-occupied eastern bank, where the worst flooding occurred.

ArcelorMittal Kryvyi Rih PJSC, the Ukrainian unit of steelmaker ArcelorMittal SA, said by email it had temporarily paused steel and rolled metal production to limit water consumption at its plant in Kryvyi Rih, about 70 kilometers (43 miles) northwest of the reservoir.

International outrage could, if sustained, make the dam’s destruction just the latest in a series of alleged and proven Russian atrocities – from the mass graves of Bucha, to the bombed-out shelters and hospitals of Mariupol – that have at critical moments stiffened US and European resolve to cross red lines and give Ukraine more of what it wants to fight back.

Hours before the breach, top diplomats from France and China had discussed the need “to begin the political settlement process,” the Foreign Ministry in Beijing said.

On Wednesday, China’s permanent representative at the United Nations, Zhang Jun, expressed his nation’s “grave concern” over the disaster, portraying it as further evidence of the need to for peace talks.

In addition to a path to NATO membership, which requires unanimity and is not expected to be granted at a summit in Vilnius next month, Ukraine is pressing to receive F-16 combat jets. After long blocking the initiative, the US recently green-lit a coalition to train Ukrainian pilots to fly the aircraft. Zelenskiy told reporters Tuesday he’s already lined up pledges to deliver the planes if the US gives permission.

Russia, according to the person close to the Defense Ministry in Moscow, believes Ukraine has until October to make its offensive count, with the likely thrust coming from Zaporizhzhia, in southeastern Ukraine, toward the south and ultimately Crimea. The assumption was that smaller strikes across the Dnipro would aid that effort, the person said in written replies sent just before the dam’s breach.

A senior NATO official gave a similar assessment of likely Russian concerns about Ukraine using the Dnipro as a secondary front.

Officials in Moscow estimate that Ukraine has prepared as many as 20 new army and national guard brigades for the offensive, bringing the total strength of its armed forces to between 700,000 and 1 million personnel, the person said. The difficulty for Ukraine, he added, is that training and western weapons supplies have been too slow, causing delays.

Russia should have enough troops, aided by air superiority, to repel the Ukrainian attack and create a stalemate, the person said, even if the record of Russian command failures leaves room for doubt.

The flooding from the dam breach will force Russian troops near the Dnipro to retreat, which may reduce the intensity of shelling against territories Ukraine controls, according to Ukrainian southern military spokeswoman Nataliya Humenyuk.

“I don’t think it’ll impact the counter-offensive as such, but I think the aim is to impact Ukrainian public’s willingness to keep up their efforts,” said Elisabeth Braw, a senior fellow at the American Enterprise Institute, where she focuses on defense against hybrid and gray-zone threats. “It’s aimed at terrorizing the Ukrainians at the very moment when they might feel optimistic about the counter-offensive.”

(Updates throughout including comment from President Zelenskiy, Arcelor Mittal and China.)

Author: Marc Champion, Alberto Nardelli and Natalia Drozdiak