AMNA NAWAZ: Welcome to the "News Hour."

A thousand days ago, Russia launched its full-scale invasion of Ukraine.

And, today, Ukraine marked that grim milestone with a first, using a long-range American missile system to hit a target inside Russia.

Also today, Russia released its updated nuclear doctrine that says it could use nuclear weapons if attacked by a conventionally armed country supported by a nuclear power, all this as leaders of the G20 nations concluded a summit in Rio de Janeiro.

That is where Nick Schifrin is covering President Biden's final gathering and Ukraine's strike -- Nick.

NICK SCHIFRIN: Amna, Ukraine fired those long-range missiles known as ATACMS, the Army Tactical Missile System, into the Russian region of Bryansk.

That is right next to the Kursk region, which Ukrainian soldiers seized this summer.

North Korean troops have recently joined Russian troops there in massing to try and recapture Kursk from Ukraine.

The target of today's attack strike is reportedly an ammunition depot.

In the coming days, expect strikes on similar targets, including logistics hubs and command-and-control sites, in order to protect those Ukrainian troops in Kursk and also make Russia pay a price for deploying North Korean troops.

Now, Ukraine is certainly a topic here, the G20 in Rio, but many of the leaders here are looking beyond President Biden, some to China, all to his successor.

In front of Rio's Sugarloaf Mountain, the world's most powerful leaders prepared for their family photo.

Chinese President Xi Jinping was there.

The collected staff was there, as were the Global South's three core leaders, Brazil, India, South Africa, next to the man who calls himself their partner.

But that's when the leaders began to notice something, someone was missing.

WOMAN: Behind the palm tree right now.

NICK SCHIFRIN: The White House pool camera hunts for the missing president, the Canadian and Italian prime ministers left to look on, as Brazil's President Lula da Silva decides not to wait.

If a picture is worth 1,000 words, it seems like the Global South doesn't need the U.S. to fill the family.

For years, Latin America has increasingly dealt with China, the largest trading partner outside the region, and often the builder of much-needed infrastructure like the Pacific Ocean's second largest port inaugurated in Peru last week.

But this week's G20 reunion was shadowed by a once and future relative.

Donald Trump was like the Harry Potter character Voldemort, he who must not be named.

JUSTIN TRUDEAU, Canadian Prime Minister: As we look to deepen our relationship, we will obviously talk about the new American president and some of the work we're going to do together around that.

NICK SCHIFRIN: There's a lot of work to be done together, from poverty to hunger to climate change.

But the next Trump administration's priorities don't necessarily align with this group, including Trump's vow to impose global tariffs, as alluded to by French President Emmanuel Macron.

EMMANUEL MACRON, French President (through translator): Fragmentation and fracture of the international order by tariff policies that are controlled by the strongest simply leads all others to not respect it.

NICK SCHIFRIN: But others show Trump more regard.

Last Thursday, Argentinean President Javier Milei visited Mar-a-Lago.

And at the G20 just days later, Milei reversed Argentina's previous position and blocked host Brazil's attempt to tax the super rich.

Milei has been an unabashed Trump supporter.

JAVIER MILEI, Argentinean President (through translator): The global elite cries over the defeat of communist Kamala Harris, yes, the communist Kamala Harris.

Meanwhile, freedom advances and expects with hope, just like other decent people, a second presidency of Donald Trump.

NICK SCHIFRIN: In its final months, the Biden administration is doing what it can to enact enduring policy change.

This weekend, President Biden toured the Amazon, the first sitting president to do so.

He created an International Conservation Day and pledged new financial help for the Amazon.

His administration hopes that investments in clean energy infrastructure are irreversible.

JOE BIDEN, President of the United States: It's true.

Some may seek to deny or delay the clean energy revolution that's under way in America.

But nobody, nobody can reverse it, nobody, not when so many people, regardless of party or politics, are enjoying its benefits.

THAIS BANNWART, Greenpeace Brazil: It is important that he went there to see with his own eyes, but he's leaving the administration, so I don't know what we can expect in concrete measures.

NICK SCHIFRIN: Thais Bannwart is with Greenpeace Brazil.

THAIS BANNWART: The policies are reversible.

And the big problem is that their consequences are more difficult to reverse than the policies itself.

NICK SCHIFRIN: So, as the G20 concluded, they retried the family photo and made sure Biden was in the middle before he says goodbye.

For the "PBS News Hour," I'm Nick Schifrin in Rio de Janeiro, Brazil.