& all that makes the continued neutrality of Sweden problematic. Both side are going to be trying to leverage Sweden to their advantage. If the Germans are to return to invading Norway in 1941 or 1942 or any time is just too tempting to badger the Swedes into cooperation, or simply invade them as well to gain any strategic advantage thereof.
As I see it, Sweden can be invaded only in spring 1941 before Barbarossa. The German strategic objective will be the subjugation of the entirety of both Sweden and Norway, up to Kiruna and Narvik. A naval campaign like the one in April 1940 will be completely ASB, so the only choice is a land campaign. A land campaign would involve a landing in Scania and then driving north. The fight in Sweden wouln't be a cakewalk:
- The Swedish had in 1941 a total of 600,000 men in a full mobilization ( 200,000 of them would have not finished their training), with an OOB of 10 divisions.
- Decent amounts of field and AA artillery but with few tanks (135 light tanks)
- The Swedish navy was capable to protect Stockholm and can hurt the Kriegsmarine, especially after the losses they have sustained in the 1940 campaign.
All in all, nothing to sneeze at, especially in a campaign in their home turf. After the Wehrmacht has beaten the Swedish army, they will have to drive to Oslo and then start a grinding campaign over mountain terrain to reach Narvik. The Wehrmacht will have to face a fully mobilized Norwegian army of 6 light divisions (that had 1 year to properly train) and whatever British and Canadian divisions are stationed there.
A Nordic Campaign will be a major one that I cannot see happening after Barbarossa has been commenced. The only problem will be the Greco-Italian War. If indeed happens, the British will be in Greece in November 1940. The RAF may threaten both the Romanian oil and the Swedish iron ore. However, I think the British won't overextend before the Americans enter the war, so I believe they would respect swedish neutrality if only to avoid fighting in Scandinavia with open fronts in Cyrenaica and Greece. Furthermore, Greece is an open sore that cannot be squashed by Mussolini and constitutes a bridgehead that allows anti-German feelings to fester in the Balkans (think of the Yugoslavian coup). Lastly, with a reduced Axis threat, Britain can buy more and more turkish chrome.
While it is still possible for Hitler to choose Scandinavia over Balkans, I would bet on a Balkan campaign.
In general, I think Scandinavia will be spared of a land campaign, at least until USA has entered the war
and has build a significant force in Norway both in terms of air power and land units. If then the Allies bully the Swedish not to sell iron to the Germans, then all bets are off. But by that point Wehrmacht will be struggling along the Volga and Caucasus...