AMNA NAWAZ: Now to our other major story, one year on since Russia's invasion of Ukraine.

The two men in charge of the world's law largest nuclear arsenals faced off rhetorically today, President Biden in Warsaw before a rousing crowd, and Russian President Putin in Moscow in front of an orderly audience of his lieutenants and supporters.

Again with the support of the Pulitzer Center, Nick Schifrin reports from Ukraine.

JOE BIDEN, President of the United States: Hello, Poland!

(CHEERING AND APPLAUSE) NICK SCHIFRIN: In an Eastern European capital neighboring the largest war in 80 years, President Biden today spoke in the language of civilizational conflict.

JOE BIDEN: The principles and the stakes are eternal, a choice between chaos and stability, between building and destroying, between hope and fear,between democracy that lifts up the human spirit and the brutal hand of the dictator who crushes it.

NICK SCHIFRIN: U.S. officials said before the speech President Biden would not get in a tit for tat, but he clearly targeted one man.

JOE BIDEN: The West was not plotting to attack Russia, as Putin said today.

This war was never a necessity; it's a tragedy.

President Putin chose this war.

Every day the war continues is his choice.

He could end the war with a word.

NICK SCHIFRIN: Poland was among the first Eastern Bloc countries to join NATO.

Today, Biden reiterated the U.S.' commitment to NATO's collective defense and vowed democracy would prevail over autocracy.

JOE BIDEN: President Putin's craven lust for land and power will fail.

And the Ukrainian people's love for their country will prevail.

Democracies of the world will stand guard over freedom today, tomorrow, and forever.

NICK SCHIFRIN: Eleven hundred miles east and eight hours earlier, President Putin had his own audience and blamed the war in Ukraine mostly on the West.

VLADIMIR PUTIN, Russian President (through translator): The responsibility for fomenting the Ukrainian conflict, for its escalation and for the increasing number of victims lies entirely with Western elites, and, of course, the current regime in Kyiv, for which the Ukrainian people are essentially strangers.

NICK SCHIFRIN: Putin announced Russia was suspending participation in the New Strategic Arms Reduction Treaty, or New START.

It's the final bilateral arms control treaty that applies to long-range city-destroying nuclear weapons.

New START limits Russia and the U.S.' deployed nuclear warheads and includes extensive verification measures.

U.S. officials had already accused Russia of blocking some of those measures.

And, tonight, the Russian Foreign Ministry said it would, in fact, respect New START requirements.

But in his 100-minute-long speech today, Putin also said he would put nuclear systems on what he called combat duty.

VLADIMIR PUTIN (through translator): What is important is that Russia-U.S. relations have deteriorated.

The responsibility for that is fully with the United States.

It was them who started revising the post-World War II arrangements after the collapse of the Soviet Union.

They started to build the world in the American way, with just one master, one lord.

NICK SCHIFRIN: Today, NATO Secretary-General Jens Stoltenberg said Putin's announcements had made the world a more dangerous place.

JENS STOLTENBERG, NATO Secretary-General: With today's decision on New START, the whole arms control architecture has been dismantled.

I'm calling on Russia today to reconsider its decision to suspend its participation in the New START agreement.

NICK SCHIFRIN: But what is clear, Putin is not reconsidering the war in Ukraine.

Today, Russian rockets struck in the middle of one of the largest cities in Ukraine's south.

At least six died in a war that Biden today called Putin's choice and Putin called a necessity.

As for President Putin's response to President Biden speech, we will have to wait for the morning for that.

But, earlier today, Putin's spokesman said that Putin wouldn't even watch Biden's speech and would instead rely on a summary of it prepared by his staff -- Amna.

AMNA NAWAZ: Nick Schifrin once again in Ukraine for us tonight.

Nick, thank you.