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Jan Henningsson said the need for Sweden to sign up to the alliance was greater than previously considered as two decades of cuts have shrunk the military to almost a 10th of its post-Cold War size.
He also suggested that if Sweden failed to sign up to the alliance this month the country would risk “back-stabbing” its neighbour Finland by allowing Russia in through the back door.
Sweden enters a crucial and historic week with its parliament debating whether it should join Nato following Russia’s invasion of Ukraine.
The proposal has led to a difficult moment for a country that has remained steadfastly neutral for more than 200 years, is resolutely anti-nuclear weapons and whose population is divided over Nato.
But Mr Henningsson, 71, believes with the army shrinking from 15 brigades in 1995 to two currently, it needs Nato membership urgently given President Vladimir Putin’s unpredictability.
“Our armed forces are not strong enough to defend our territory on their own,” he told The National in his home city of Uppsala. “It's become obvious that the dismantling of the Swedish defence forces has gone too far, that we would actually be at risk if we were attacked.”
Just after the Cold War Sweden had a standing army of nearly 100,000 personnel but that had shrunk to 15,000 in 2018.
Conscription was reintroduced to bring numbers back up but currently it can only field 23,000 troops from a population of nearly 11 million, despite having an excellent defence industry that has world-leading hardware, including Gripen fighters.
“This makes it even more important for Sweden to co-operate and to join forces with other countries in order to defend ourselves,” said Mr Henningsson. “The best guarantee for us would be Nato.”
If Sweden opted against joining it would be the only Nordic country outside the alliance. Norway, Denmark and Iceland are already members.
Much depends on the ruling Social Democrats’ decision — whose women and youth wings are largely against Nato — which will announce an executive decision on Sunday. If it is “no” that would have significant ramifications.
With fears of Russian interference in any referendum the decision will be down to a simple majority in parliament, but for long-term stability it is thought a broad majority would be politically necessary.
“If Sweden stays out of Nato it will open up a free area for Russia to backstab not only Finland but also the Baltic states,” said Mr Henningsson, who was a senior adviser in the Middle East and North Africa division. “The Russians could also take the strategically vital island of Gotland and you can see from the geography that presents a great danger for Finland and the Baltic states.”
The regiment on the island in the centre of the Baltic Sea has recently been reinforced but it could not “hold Gotland for very long”.
Sweden and Finland were united for 600 years until 1809 and there remains a strong emotional bond between the two countries, especially among the older generation whose relatives volunteered to fight for the Helsinki government when Russian invaded in 1940.
There is also an understanding that joining Nato move would help bolster the Baltic States, which are largely surrounded by Russia, and help to soothe “great shame” over the post-Second World War extradition of Baltic volunteers to the Soviet Union who faced almost certain death. “We would not want to let down the Baltic people a second time,” he said.
Mr Putin’s invasion of Ukraine has “turned the tables completely” on both Sweden and Finland’s neutrality, he said.
“The Russian Federation could previously be counted upon as being rational and sane, but what Finland has discovered with horror is that its neighbour suddenly turned out to be an aggressor with no compunction whatsoever,” said Mr Henningsson, who was director of the Swedish Institute in Alexandria, Egypt, from 2002 to 2008.
He suggested an initial downside of joining Nato would only be that Swedish officers would be sent to its headquarters in Brussels, “further depleting the resources of military staff in Sweden”.
However, there is hope that its military will rebuild after the government has passed a bill to increase its defence spending by 40 per cent to $11 billion by 2025, reaching the Nato target of 2 per of gross domestic product (GDP).
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