Sprayed between Sweden and Finland, the autonomous Aland Islands are a picturesque archipelago once part of Russia and demilitarised since 1856.

But the region's unique status is the object of intense debate since Russia's invasion of Ukraine rattled neighbouring Finland into applying for NATO membership in May.

Under international treaties signed after the Crimean War, no troops or fortifications can be placed on the strategic Baltic Sea islands.

"It is the Achilles' heel of Finland's defence," Alpo Rusi, a professor and former presidential advisor, told AFP.

Home to about 30,000 mostly Swedish-speaking Finns, the area is characterised by rocky islands, lush green forests, old stone churches and wooden architecture -- all under the watchful eye of a Russian consulate.

"We have always thought, 'Who would want to attack us when we have nothing worth taking?'," 81-year-old Ulf Grussner told AFP.

The region's unique status is the object of intense debate since the Russian invasion of Ukraine
The region's unique status is the object of intense debate since the Russian invasion of Ukraine AFP / Alessandro RAMPAZZO

"But that has changed with Putin's war on Ukraine", said the pensioner, one of many here who want Aland to remain demilitarised.

In June, a poll showed 58 percent of Finns would approve of a military presence on Aland, which celebrated the 100th anniversary of its autonomy on Thursday.

"There is concern over whether Finland could react fast enough militarily in the event of a sudden intrusion on Aland," Rusi said.

Armies wrestled for control of the archipelago in both World Wars.

"Why should we trust the idea ... that troops would not rush to control Aland as fast as possible," said Charly Salonius-Pasternak, a researcher at the Finnish Institute of International Affairs.

The Russian consulate in the main town of Mariehamn
The Russian consulate in the main town of Mariehamn AFP / Alessandro RAMPAZZO

Alanders, on the other hand, are keen to protect their special status and have so far firmly rejected the idea of ending the demilitarisation.

"Why should we change it? I think it's a stabilising factor in the Baltic Sea area that we are demilitarised," Veronica Thornroos, 59, premier of the Aland government, told AFP.

Besides, if the archipelago were attacked, Finland would defend it "very quickly", she said.

The Finnish government has said it has no intention of touching Aland's special status.

Sia Spiliopoulou Akermark, director of the Aland Peace Institute, meanwhile noted that the "Aland regime" of autonomy, cultural guarantees and demilitarisation is a "complex knot" that should be considered as a whole.

Ulf Grussner's idyllic childhood home was taken by the Russians
Ulf Grussner's idyllic childhood home was taken by the Russians AFP / Alessandro RAMPAZZO

Like the rest of Finland, Aland was part of the Russian empire from 1809 to 1917.

At the time, the archipelago was viewed as an important outpost in the defence of Saint Petersburg and control of the Baltic Sea.

Finland gained independence from Russia in 1917, and was granted sovereignty over Aland in 1921 despite protests from the islands' Swedish-speaking majority.

The Nordic country went on to fight two bloody wars against the Soviet Union during World War II.

As part of their peace deal, the demilitarisation of Aland was to be monitored by a Soviet consulate in the archipelago's main town of Mariehamn.

The consulate still exists to this day, although it is now run by Russia.

A group of locals gather every day outside the high metal fence protecting the consulate, to protest Russia's war in Ukraine.

"They have no business being here. Russia is always a threat", one of the protestors, Mosse Wallen, 71, told AFP.

Russia also owns a seaside property north of Mariehamn in Saltvik, which was acquired in the 1947 peace deal.

"They gave my mother three days to move out", said Ulf Grussner, whose idyllic childhood home is now fenced in by the consulate.

Grussner's father was a German geologist, and the peace deal stipulated that all German possessions in Finland were to be ceded to the Soviets.

In 2009, ownership of a piece of the property was transferred to the Russian presidency.

Concern has mounted in Finland in recent years over Russian property deals across the country.

Grussner feared that Russia might intend to use his family's property and the demilitarisation as a "pretext" to increase its presence in the area.

"It is far-fetched, but on the other hand it's not impossible," he said.