He Knows Putin Well. And He Fears for Ukraine.

The Finnish president, Sauli Niinisto, has carved out a vital role as interpreter between East and West, and he is not optimistic about the prospects for peace.

HELSINKI — As the threat of a new Russian invasion of Ukraine grew, the European head of state with the longest and deepest experience dealing with Vladimir V. Putin fielded calls and doled out advice to President Emmanuel Macron of France and other world leaders desperate for insight into his difficult neighbor to the east.

“‘What do you think about this about this, what about this, or this?’ That’s where I try to be helpful,” said Sauli Niinisto, the president of Finland, as the harsh light gleaming off the snow and frozen bay poured into the presidential residence. “They know that I know Putin,” he added. “And because it goes the other way around Putin sometimes says, ‘Well, why don’t you tell your Western friends that and that and that?’”

Mr. Niinisto, 73, said his role was not merely that of a Nordic runner, shuttling messages between East and West, but of borderland interpreter, explaining to both sides the thinking of the other. The departure from politics of Angela Merkel, who for years as Germany’s chancellor led Europe’s negotiations with Mr. Putin, has made Mr. Niinisto’s role, while smaller, vital, especially as the drumbeat of war grows louder.