President Joe Biden said Wednesday that Russian President Vladimir Putin made a “big mistake” by suspending his country’s participation in the the last remaining U.S.-Russia nuclear arms control treaty. The U.S. president was in Poland to reassure eastern flank NATO allies that the U.S. will remain by their sides amid the grinding Russian invasion of Ukraine.

In his first comments since Putin’s announcement Tuesday, Biden condemned the Russian decision to pull back from the treaty, known as New START. The move is expected to have an immediate impact on U.S. visibility into Russian nuclear activities, but the pact was already on life support following Moscow’s cancellation late last year of talks that had been intended to salvage an agreement that both sides have accused the other of violating.

“It’s a big mistake,” Biden said.

The president’s comments came as he was wrapping up a whirlwind, four-day visit to Poland and Ukraine with talks with leaders from the Bucharest Nine, a collection of nations in the most eastern parts of the NATO alliance that came together in response to Putin’s 2014 annexation of Crimea from Ukraine.

As the war in Ukraine drags on, the Bucharest Nine countries’ anxieties have remained heightened. Many worry Putin could move to take military action against them next if he’s successful in Ukraine. The alliance includes Bulgaria, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Romania and Slovakia.

“You’re the frontlines of our collective defense,” Biden said Wednesday of the group. “And you know, better than anyone, what’s at stake in this conflict? Not just for Ukraine, but for the freedom of democracies throughout Europe and around the world.”

He pledged that NATO’s mutual-defense pact is “sacred” and that “we will defend literally every inch of NATO.”

A day earlier at the foot of Warsaw’s Royal Castle to mark the somber milestone of the year-old Russian invasion, Biden warned that Russian aggression, if unchecked, wouldn’t stop at Ukraine’s borders. “Appetites of the autocrat cannot be appeased,” he said. “They must be opposed.”

Hungarian Prime Minister Viktor Orban, the right-wing populist leader who argued last week that the European Union is partly to blame for prolonging Russia’s war in Ukraine, has balked at sanctions on Moscow and arming Kyiv. Orban was skipping the meeting with Biden, and President Katalin Novák was attending in his stead.

Still, Klaus Iohannis, the president of Romania, insisted that “The B9 is stronger than ever.”

NATO Secretary General Jens Stoltenberg, who was attending the meeting, said, “We don’t know when the war will end, but when it does, we need to ensure that history does not repeat itself.”

‘Cycle of Russian Aggression’

Pointing to past Russian actions in Georgia and Ukraine, he added, “We cannot allow Russia to continue to chip away at European security. We must break the cycle of Russian aggression.”

Biden met Tuesday in Warsaw with Moldovan President Maia Sandu, who last week claimed Moscow was behind a plot to overthrow her country’s government using external saboteurs.

Sandwiched between Ukraine and Romania and one of Europe’s poorest countries, the Eastern European nation has had historic ties to Russia but wants to join the 27-nation European Union. Biden in his remarks endorsed Moldova’s bid to join the EU.

“I’m proud to stand with you and the freedom-loving people of Moldova,” Biden said of Sandu and her country in his Tuesday address.

Since Russia invaded Ukraine nearly a year ago, Moldova, a former Soviet republic of about 2.6 million people, has sought to forge closer ties with its Western partners. Last June, it was granted EU candidate status, the same day as Ukraine.

Sandu spoke out last week about a Russian plot “to overthrow the constitutional order.” She spoke out after Ukrainian President Volodymyr Zelenskyy said his country had intercepted plans by Russian secret services to destroy Moldova. Those claims were later confirmed by Moldovan intelligence officials.

Biden’s speech on the Ukraine war came one day after he made a surprise visit to Kyiv, a grand gesture of solidarity with Ukraine. The address was part affirmation of Europe’s role in helping Ukraine repel Russia’s ongoing invasion and part sharply worded warning to Putin that the U.S. won’t abide Moscow defeating Ukraine.

The White House has praised several eastern flank countries, including Lithuania, Poland and Romania, over the last year for stepping up efforts to back Ukraine with weapons and economic aid and taking in refugees.

Biden has given particular attention to Poland’s efforts. The country is hosting about 1.5 million Ukrainian refugees and has committed $3.8 billion in military and economic assistance to Kyiv.

“The truth of the matter is: The United States needs Poland and NATO as much as NATO needs the United States,” Biden said during talks with Polish President Andrzej Duda.