STOCKHOLM — Swedish neutrality could be consigned to the history books after two centuries as the perceived threat from Russia, complete with warplanes flying overhead and a mystery submarine in the Baltic, prompts one of the world's most pacifist countries to re-militarize.

In an election year, growing support for joining NATO is putting the center-left government of Prime Minister Stefan Löfven under pressure, with the four-party opposition bloc now united in backing membership of the military alliance for the first time.

They received encouragement from NATO chief Jens Stoltenberg, who said during a visit to Sweden earlier this month: “Sweden and NATO cooperate very tightly."

"If Sweden were to apply to join, I think there would be broad support for that within NATO,” he later added.

The country has already reintroduced military service, which was scrapped in 2010, for a limited number of draftees who will be called up for compulsory basic training this year. It has decided to station troops on the Baltic island of Gotland for the first time in a decade, and boosted military spending by 2.7 billion Swedish kronor (€274 million) a year from this year through 2020.

"If Sweden were to apply to join, I think there would be broad support for that within NATO” — NATO chief Jens Stoltenberg

The new emphasis on military preparedness follows a series of security incidents involving Russia. In 2014, Moscow’s decision to annex Ukraine’s Crimean peninsula spooked its neighbors in the Baltic Sea region, who feared that the border violations could become part of a wider pattern.

Also in 2014, Swedish authorities confirmed that an unidentified submarine had entered territorial waters close to Stockholm. While a number of lawmakers accused Russia, no evidence linking the incident to Moscow was produced and Russia denied involvement.

Repeated incursions by Russian aircraft into Swedish airspace also rattled nerves in Stockholm and led to complaints to the Russian ambassador here.

At the same time, political support for NATO membership has been rising. In 2015, two center-right parties, the Center Party and the Christian Democrats, changed their minds on NATO and joined their opposition allies, the Moderate Party and the Liberal Party, in advocating membership.

Public opinion turns

Liberal Party defense spokesman Allan Widman said he could now see an opportunity for a serious debate on the subject ahead of September's national election.

“For the first time ever, you have four parties in one bloc that are all in favor of NATO,” he told POLITICO in an interview. “I think the upcoming election could be something of a referendum about NATO membership for Sweden.”

Public opinion is also shifting.

In a survey this month by the pollster Inizio for the daily Aftonbladet, 43 percent of Swedes polled said their country should join NATO, versus 37 percent who opposed it (and 20 percent who didn't know). That is a turnaround from 37 percent in favor and 42 percent against in a similar poll 12 months ago.

Still, political resistance to joining NATO remains compact among the left-leaning parties that currently run the country in a minority coalition.

Sweden’s current Social Democrat leader, Löfven, has consistently said his country has no plans to join NATO and that its freedom from alliances has served it well. Foreign Minister Margot Wallström has suggested joining NATO would be an unnecessary provocation of Russia at a time of increased tension.

The far-right Sweden Democrats are also against joining NATO, a factor that would make the center right keener to get the Social Democrats on board before filing the paperwork for membership, experts say.

“If Sweden is going to seek NATO membership, the Social Democrats also have to accept it,” said Ulf Bjereld, a political scientist at Gothenburg University and an active member of the Social Democrats.

'We won't budget a millimeter'

Sweden’s history as a non-aligned nation has deep roots. It has not fought a war for over 200 years, avoiding direct involvement in the two world wars and refusing to officially join either side during the Cold War.

Under leaders like the Social Democrat Olof Palme (who served two terms as prime minister and was assassinated in 1986), Sweden was a vocal critic of U.S. and NATO policies at times, even as the country sought to maintain strong relations with Washington. It set itself up as an independent voice between NATO and the USSR, believing it could help ease tensions.

“We won’t budge a millimeter when it comes to asserting our neutrality and Sweden’s independence which has given us peace for 170 years,” Palme told parliament in 1985.

Since the 1990s Sweden has moved steadily closer to NATO and it now has what are called enhanced opportunities for dialogue and cooperation with NATO, along with just four other non-members.

It contributed to NATO-led operations and missions in the Balkans and Afghanistan and trains regularly with NATO forces. It agreed to provide Host Nation Support in 2016, which is a commitment to make it more straightforward for NATO forces to operate from Swedish territory should they need to.

Comments by Moderate Party leader Ulf Kristersson to Swedish media suggested that even if the center-right bloc were to secure a majority in parliament at the September elections, he would likely seek to win over the Social Democrats before applying to join NATO.

However, he also said that “a large minority should not have a veto forever.”

Opinion polling suggests a minority center-left or minority center-right government are the most likely outcomes in September, with little to pick between them at present.

Kristersson claims that without recourse to the collective defense guarantee offered by NATO, his country remains vulnerable.

“We don’t have the guarantees a real member would have if something were to happen,” he said.