Image copyright Getty Images Image caption President Obama, shown in Stockholm in 2013, has invited Nordic leaders to dinner

The leaders of five Nordic countries are meeting President Barack Obama for a summit at the White House. They'll talk about the environment, refugees - and the possibility of a future conflict with Russia.

President Obama is hosting the White House's first-ever US-Nordic Leaders Summit on an unlucky day, Friday the 13th. But Nordic officials said that they're not superstitious.

The agenda includes discussions about the refugee crisis and Nordic contributions to the fight against the Islamic State group. Swedish officials are faced with an influx of a "very, very high" number of refugees, said Ambassador Björn Lyrvall.

Meanwhile Denmark, as Lars Gert Lose, their ambassador to the US, told me, is sending 60 special forces to Iraq and Syria.

Image copyright Getty Images Image caption President Putin, shown in Finland in 2013, has unnerved some people in Nordic countries

On Friday evening, the Nordic officials will attend a state dinner at the White House.

State dinners are relatively rare. Since moving into the White House, the Obamas have hosted less than a dozen - for the leaders of India, China, the UK and a handful of other countries.

The Nordic guests seem a little surprised by the attention.

But they also know - in a visceral way - why they've become important to Americans. Russians are now acting out in northern Europe in a way that hasn't been seen since the Cold War.

For this reason, said the Center for Strategic and International Studies' Heather Conley, a former US state department official, the relationship between the US and Nordic countries "has been taken to a new level".

The message from President Vladimir Putin is dark.

Russian aircraft flew over the Baltic Sea last month, zooming towards a US ship. Russian submarines may be lurking off in the waters near Sweden and Finland.

Last year a Russian ambassador said that if Danish leaders stepped out of line - by joining Nato's missile defence system, for example, the Russians would fire nuclear missiles at their ships.

Image caption Georgetown University's Angela Stent said US officials take Russians' rhetoric seriously

Nordic leaders worry most about their neighbours, Estonia, Latvia and Lithuania. These three countries are located across the Baltic Sea from Sweden - and are close to Russia.

They could be in danger. It wasn't that long ago that Putin reclaimed Crimea, saying that it was part of Russia. Some analysts in the Baltics and Eastern Europe fear he could try the same thing with Estonia, Latvia or Lithuania.

Putin has changed the dynamics of the region - and is partly why White House officials have organised the Nordic summit.

"You can't assume it's just rhetoric," said Georgetown University's Angela Stent, who writes about US-Russia relations. She said White House officials are taking the Russian statements seriously.

Administration officials said that the summit was convened as a way to "deepen US-Nordic cooperation". They also said it shows their commitment to the security of the Nordic region.

Image caption Ambassador Lars Gert Lose of Denmark described an "unpredictable" Russia

At the summit US officials will talk to their Nordic counterparts about dealing with an "unpredictable" Russia, as Lose said, diplomatically.

University of Colorado's Benjamin Teitelbaum, the author of a book about the Nordic nations, said officials have been surprised at Putin's aggression.

Teitelbaum said: "This is a traumatic thing."

So traumatic, in fact, that some Swedes are now talking about joining Nato. Sweden is a partner of Nato, but it's not officially a member.

Over the years, Swedes have become more open to the idea of joining the alliance. In 1994 only 15% of Swedes wanted to become Nato members, according to SOM Institute, an independent organisation at the University of Gothenburg.

Last year, according to researcher Karl Yden, nearly 40% of Swedes supported the prospect of membership.

They're interested - but at this point nothing will change.

Officials in the Swedish government, a coalition of left-leaning parties, have no plans to apply for Nato membership, said Ambassador Björn Lyrvall.

He spoke with me at the Swedish embassy, located in a building near the Potomac River. His colleague, Monica Enqvist, offered me coffee and a plate of havreflarn (oat crisps).

A television set was turned to MSNBC, muted. The TV is anchored on a wall near an Ivan Agueli painting of a field and blue-grey sky.

"It's contemplative," said Lyrvall, who has sandy-coloured hair that falls over his forehead. As he talked about Russia, he looked outside at the rain.

"Swedish weather," said Enqvist, later.

Image caption Ambassador Björn Lyrvall said that some Swedes are considering Nato membership

A senior administration official said he planned to reassure his Nordic counterparts at the summit about their security and said he'd deliver an upbeat message: "Yes, the US stands by its commitment to your security."

The official reminded me of the reassuring remarks that the president had made years ago in the Baltic region.

"We'll be here for Estonia. We will be here for Latvia. We will be here for Lithuania," Obama said in Tallinn, Estonia, in September 2014. "You lost your independence once before. With Nato, you will never lose it again."

It's a noble sentiment - but perhaps not realistic. A new study by researchers at Rand Corporation, a US think-tank, showed that if Russia attacked the Baltics, Nato wouldn't be able to defend them.

Administration officials said they've been beefing up security in the region and are working with allies - and that the summit is part of their effort.

Late on Friday morning, Obama and the Nordic leaders will speak briefly with reporters. They'll issue a declaration with "substantial language", said Lyrvall, about Russia.

In the evening they'll have dinner on the south lawn - "a grand event", he said. Obama will make a toast, and so will leaders of Denmark and Iceland.

Lyrvall may be an introspective - and brooding - Swede. But he said that he pays no attention to unlucky signs, adding: "I think we have a strong enough relationship to withstand the 13th of May."

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