"I cannot get over the feeling that something terrible is going to happen to this city," said Mystislav Chernov, in his documentary, "20 Days in Mariupol."

Something terrible did happen to the Ukrainian city, and Chernov, of the Associated Press, was there to witness it. "What I did not expect is that the bombardment will be so intensive," he told CBS News.

Chernov and his team spent the first 20 days of the Russian invasion inside Mariupol, sending out images that horrified the world, and are now part of an Oscar-nominated documentary. "The whole city was suffering; the whole city was starving; the whole city was without water," Chernov said. "So many people dying."

The film includes frantic efforts to save a four-year-old, and a doctor urging the camera to capture it all. "Show this Putin bastard the eyes of this child," the doctor says.

Another infant is rushed in, but he is beyond saving. There is shock and grief beyond comprehension.

As narrator, Chernov notes, "My brain will want to forget all this, but my camera will not let it happen."

To watch a trailer for "20 Days in Mariupol," click on the video player below:

The bombs started hitting schools and civilian buildings, and eventually hit the maternity hospital. One scene of a woman named Irina with her unborn child – neither of whom would live – was denounced by Russia as "staged," the women "actresses" ... a preposterous claim in the face of Chernov's unblinking camera. But with power failing and communications dwindling, he could only transmit snippets of video to the outside world.

One man tells Chernov's camera, "Russian troops commit war crimes. Our families, our women, our children need help. Our people need help from international society. Please, help Mariupol."

For the world to see what the Russians did in Mariupol, Chernov had to get his 30 hours of video out of the city. Ukrainian troops went in to rescue him. He left the city on Day 20; Mariupol fell on Day 86. Chernov said, "Believe it or not, it became even worse. It seemed that it can't be worse, just can't. But it was." 

That was the beginning of a war that has now gone on for two years.

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Thousands of Ukrainian soldiers have lost limbs. A tiny fraction of them have been brought to the United States to be fitted for advanced prosthetics.

Before the war, Oleksii Dernov was a martial arts enthusiast; Petro Kulyk worked in construction; Levgen Kaliuzhnyi was a lumberjack; and Serhii Volyk repaired cars.

They tell of being outgunned and outmanned by the Russians.

Asked how heavy Russian artillery fire was, Kaliuzhnyi replied, "In ratio to our artillery, it's one to ten."

Dernov didn't know by how much Russians outnumbered Ukrainians, "but there's lots."

Kulyk added, "They come and come and come."

The combat was so intense there was no quick evacuation from the front lines. It took Kaliuzhnyi more than 24 hours to get to a hospital, Kulyk around 20.

These soldiers were wounded in Ukraine's much vaunted counter-offensive – last year's drive to take back occupied territory, a drive that attempted to use American mine clearing equipment and armored vehicles to break through Russian lines, but faltered in the face of mine fields and dug-in defenders.

Now the American supply line which sent Ukraine over three million artillery shells is drying up, and funding for more military aid is trapped in the caustic politics of Washington.

President Joe Biden had a message to those who were blocking military aid: "For Republicans in Congress who think they can oppose funding for Ukraine and not be held accountable, history is watching."

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In a floor speech, Senator Angus King (I-Me.) said, "When the history of this day is written, as it surely will be, do you really want to be recorded as being on the side of Vladimir Putin? All those in favor of Putin say aye."

King, who has traveled to Ukraine and met with its leader, said what happens next will be a turning point: "If we walk away, it will be the greatest geopolitical mistake this country has made in generations, and it will haunt this country for 50 years."

Serving on both the Senate Armed Services and Intelligence Committees, King paints a dark picture of what will happen to Ukraine without U.S. aid: "There'll be one of two results: Russia will just take over and the Ukrainian people's desire for freedom and democracy is gone; the other option is that it turns into a sort of guerilla war where Ukrainians are fighting from behind trees and buildings."

The inevitable conclusion, said King, is that in about six months the front lines as we know them today will collapse and the Russians will break through. "I think that's a distinct possibility," he said.

In his documentary "20 Days in Mariupol," Chernov captures the battle for survival in one unforgettable scene - a wounded mother giving birth.

"They got the child out and the child was silent," he said. "It was this tension - I've never felt anything like that before. No one would be able to bear one more child's death. ... and then she screamed, and everyone was in tears."

He called it the defining moment of the whole horrifying 20 days. "One of the doctors told me that people were only dying in this room, and this is the first time a human was born in this room," Chernov said.

The baby whose birth we witnessed is coming up on her second birthday. The war is coming up on its second birthday, too. More cities and towns have fallen since Mariupol, and many more are under attack.

Chernov said, "All of those cities, their story is represented by Mariupol. It's a symbol of all these cities. It's not the past; it's present."

      
For more info:

• "20 Days in Mariupol" will be presented on the PBS series "Frontline" February 22, and can be streamed on pbs.org
• Future for Ukraine

     
Story produced by Mary Walsh. Editor: Joseph Frandino. 

      
See also: 

In:
• Ukraine
• Russia

David Martin

David Martin is CBS News' National Security Correspondent.