Updated April 27, 2022 at 1:24 PM ET

In Poland, after a long train trip to Kyiv to meet Ukrainian President Volodymr Zelenskyy, Defense Secretary Lloyd Austin was blunt about U.S. goals in the conflict in Ukraine.

"We want to see Russia weakened to the degree that it can't do the kinds of things that it has done in invading Ukraine," he said Monday.

The United States and its allies now believe Ukraine can win the war against Russia. It's a significant change in thinking, bolstered by the successes of the Ukrainian military in holding its own and pushing Russia's military back from Kyiv, over the course of two months of combat.

"We believe that we can win — they can win if they have the right equipment, the right support," Austin said.

The U.S. is rushing in long-range artillery, other weapons and ammunition as the battle shifts to the east and south of Ukraine. Washington has delivered $3.7 billion in security assistance to Ukraine since Russia's full-scale invasion began in February, according to the State Department. The most recent deliveries include howitzer cannons, which Pentagon officials believe will be useful in the flat terrain of eastern Ukraine's Donbas region.

Secretary of State Antony Blinken, who also visited Kyiv last weekend, has repeatedly said the war in Ukraine would be a "strategic defeat" for Russian President Vladimir Putin and will leave Russia weakened.

This marks a shift for the U.S. in its war aims, which originally were to help Ukraine defend itself against a larger, stronger power, and now involve defeating Russia. The shift is aided by massive U.S. arms shipments to the Ukrainian forces. That raises the risk of potentially widening the conflict, analysts say. It increases the threat of a weakened Russia resorting to the use of nuclear weapons and could further destabilize an already fragile global economy.

"We don't want Russia to be a total basket case, reviving the sort of loose nukes fears of the 1990s and so on, or to become an incorrigible international spoiler," says Samuel Charap of Rand Corp.

At some point, Ukraine might want the U.S. to ease up on sanctions against Russia if that will help Kyiv reach a negotiated settlement with Moscow.

The sanctions and arms shipments could also provoke a more direct confrontation between Russia and NATO. It is difficult to know where Russia's red line is. "Nobody knows what step ... will send Putin over the edge," Charap says.

Russia says Western military aid convoys into Ukraine are "legitimate targets" but has not not managed to slow the shipments.

On Monday, Russian Foreign Minister Sergey Lavrov said no one should underestimate the possibility of a nuclear confrontation and that he viewed NATO as being engaged in a proxy war with Russia by supplying Ukraine with weaponry.

State Department spokesman Ned Price described the statement as "deeply irresponsible" and a "continuation of the Russian government's very clear attempts to distract from its failure in Ukraine."

Eventually, the U.S. and Russia have to start talking again, says Rose Gottemoeller, a former U.S. arms control negotiator who served in the Clinton and Obama administrations. "I do think at some point we are going to have to reopen some discussions with Russia, at least about constraining and controlling nuclear weapons and see where we can go from there, because it's not in our interest to have a great big pariah state with nuclear weapons," she told NPR.

For now, though, the Biden administration wants to help Ukraine defeat Russia.

Gottemoeller, a former deputy secretary general of NATO, says she's betting on "NATO's military industrial complex against the Russian military industrial complex." But she says allies need to be ready for "a big investment of resources and a big investment of time."

The Kremlin has already suffered one big loss, she says. Putin wanted NATO pushed back to its 1997 lines, before the alliance opened its doors to former Soviet bloc countries.

"Instead," she says, "he has a NATO more coherent, more together and more resolved to work together to really defeat this threat to its partner, Ukraine."

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