The president of Finland, Sauli Niinisto, is the person considered most responsible for bringing his country into the NATO alliance — and Sweden, too, which is awaiting ratification — following the Russian invasion of Ukraine. President Biden has consulted him about Russia and its president, Vladimir V. Putin, whom Mr. Niinisto has met numerous times.

In a long interview in his light-filled modernist residence in Helsinki, Mr. Niinisto warned European leaders and citizens not to become complacent over the risks of escalation in Russia’s war against Ukraine.

The war in Ukraine will last a long time, he said, and wars can take unexpected paths, even toward the use of nuclear weapons.

The invasion, Mr. Niinisto said, was “a wake-up call” for Europe and NATO.

“Well, it was ringing loudly in February 2022,” he said. “But do you hear it anymore? That clearly? That might be a good question — whether all Europeans realize that this is a European issue.”

Mr. Niinisto, 75, is nearing the end of his 12 years as the Finnish president. In the interview, he was philosophical, but troubled, too. Finland has much experience — and an 830-mile border — with its imperialist neighbor, Russia.

Recalling Finland’s numerous wars with Moscow, including the 1939 Winter War and World War II, when the Finns fought off the Soviets but had to cede some territory, Mr. Niinisto said European countries that let down their defenses after the collapse of the Soviet Union made a grave mistake.

Here are a few highlights from the interview:

Speaking about debris from what appeared to be a Russian drone landing recently in Romania, which is a member of NATO, Mr. Niinisto cautioned: “We’re in a very sensitive situation. Even small things can change matters a great deal and unfortunately for the worse. That is the risk of such large-scale warfare.” He added, “The risk that nuclear weapons could be used is tremendous.”

His warnings, he said, were partly a response to those who criticize the policies of Mr. Biden and Chancellor Olaf Scholz of Germany as too cautious in supplying Ukraine with sophisticated, long-range missiles and drones that could easily hit Russian-occupied Crimea and Russia.

“There’s a difference between those who have responsibility and those who don’t,” he said. “Also, in Finland, we hear voices that America should do that or that. And I just wanted to point out that if there’s escalation to a big war, that’s world war, so then the nuclear risk becomes clearly bigger.” He urged everyone “to understand the position of those who have responsibility.”

He urged Europeans to heed Finland’s example.

Unlike Sweden, a close neighbor in all fields, including defense, Finland still has conscription for males, and also allows women to enlist. Those who finish conscription remain in the reserves, as they do in Israel, for decades, and take part in military training and exercises at least twice a year — now more often — in conjunction with other public services, such as the police and the fire brigades.

And Finland, schooled in self-reliance, also maintains its large artillery forces, still makes its own shells and ammunition, and even bought advanced F-35 fighter jets before the Russian invasion of Ukraine.

After the Cold War, Mr. Niinisto said, “we Europeans learned to live an always improving life.”

“Decade after decade,” he said, “it strengthened the feeling that it’s a bit old-fashioned even to talk about defense forces or defending because that’s not possible in a modern world. Now there’s a huge wake-up. Fortunately, in Finland, our position remained totally different.”

In their meetings before the invasion in February 2022, Mr. Niinisto said, Mr. Putin was focused, aggressive and well informed, even obsessive, about Russian culture. He said he decided to test Mr. Putin by asking him about Mikhail Lermontov’s poem on the death of Pushkin, Russia’s greatest poet. Mr. Putin spoke for more than half an hour. “He knew everything about that — for him it’s Russia, Russia overall,” Mr. Niinisto said.

Russia ruled Finland for more than a century, until, in the chaos of Lenin’s takeover, Finland declared independence in 1917. The wars with Russia since then are seared “in our backbone,” Mr. Niinisto said. Russian history goes in waves, he said, citing “a centuries-old Finnish saying that ‘the Cossack takes anything that is loose,’” that is not tied down. (Finns used to use “Cossack” as shorthand for “Russians,” he said.) But it is a reminder that free countries must keep their defenses up and their goods safely stored.

The two presidents spoke about Russian intentions in Ukraine, beginning at the Glasgow climate summit in early November 2021, when Russian troops were building up on the border before they finally invaded in February 2022. They spoke regularly in December and in January, before Mr. Niinisto met Mr. Putin in Moscow, to urge him not to invade, Mr. Niinisto said.

After the invasion, Mr. Niinisto was among the first European leaders to meet Mr. Biden in the White House, on March 4, where he put forth the possibility of Finland joining NATO. After the Russian invasion, he said, “it became very obvious that we had no other alternative than giving up our military nonalignment.”

Mr. Biden was supportive from the start, Mr. Niinisto said.

Mr. Niinisto said he does not know how long the war will last, or how it will end, or “what life will be like when we again have peace.”

But even when the conflict ends, Russia will remain. “There’s also a big European interest to make sure that Russia is not returning back to warfare after peace in Ukraine” without insisting that the Russians “have to be blown out,” he said carefully. But he emphasized that trust would be needed to ensure that “a new war is not waiting behind the door.”

There is always life after war, he said, and there is nothing more valuable for people than peace.

“Without peace, you have nothing, so I’m sure that ordinary Russians share these feelings,” Mr. Niinisto said. “It’s a basic human feeling.”

There must be a way to maintain a relationship with Russia, he said. “I don’t mean any great friendship,” Mr. Niinisto said, “but the capability to tolerate, even understand each other a bit.”