Kallas said that Navalny's death shows that Putin's playbook hasn't changed. "This is the way he operates. This is the dictator's playbook in real life, so we should be aware of this."

But let's also learn from history, she said. "We saw it in the 1930s. The same thing. I mean, not stopping the aggressor when we had a chance to stop him, and then seeing aggression spread all over the world."

"We have already learned from the 30s and the World War II that everything spreads very fast in Europe, and also, if America isolates itself, it eventually is going to cost you more," she told Ricketts, a member of the U.S. Senate Foreign Relations Committee.

Responding to NATO Secretary General Jens Stoltenberg's discussion of the likelihood of a real threat to NATO allies, she reiterated that Ukraine's victory is the key to peace in the region. "Our focus should not drift away from helping Ukraine militarily. We don't need to talk about NATO countries if we stop Putin in Ukraine," she said.

CNN's Christiane Amanpour, who moderated the debate, asked Kallas if she fears that the U.S. might change course after the presidential election.

"I want to correct one thing," Kallas said, "I often hear the Baltic states and Poland being mentioned separately, as if we are second-class NATO members. We are all equal."

"When Russia is going to attack NATO, it is going to attack all of us; not just Poland, or the Baltic. So let's not make that mistake."

"Also, of course, we are all democracies, and we don't get to choose the leaders for our allies, and we have to work with all the allies. But we also have to learn from the mistakes of history," she said.

"I was just in the Central European countries, and they had the attitude 'the war is far from us,' so I took the map and counted kilometers, the distances: they are much closer to the war zone than we are. We should all be worried," she said.

"We have to ramp up our defense spending and do it not only in terms of political pledges but in actual life. In Estonia, we are investing over 3 percent of our GDP in defense, and we encourage everybody to do so," Kallas said.

"When I came up with the artillery initiative for Ukraine, it turned out that our [collective] defense industry is not that capable, we don't have enough," she said.

"In Estonia, for example, we have vibrant tech sectors, so why don't we put those sectors and the defense industry together to facilitate a leap forward and produce not what was there in the 20th century, but what should be there in the 21st century?" she went on.

"There is so much we can do together, and together with the big allies and the smaller ones, every one brings something to the table; that is what the alliance is all about."

At the end of the debate, Kallas poked at the U.S. senator about his migration concerns. "Just for comparison," she said, "we now have 6 percent of our population consisting of Ukrainian refugees; that would be 20 million people in the U.S.; we survive, and you will."

"The majority of the investments in the U.S. come from Europe, with 45 out of 50 states depending on Europe for job creation and export-import relations, while American companies benefit 2.7 more from Europe than from Asia."

"So if you think in terms of 'what's in it for us,' then Europe is definitely, you know, profitable for you, and that's why we should also see how it is going to affect the U.S. when there is a war in Europe," she said.

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