The Siege of Salzburg
1380 A.D.: A second town on the Southern frontier, Linköping, is founded to help bridge Turku and the rest of Sweden. Austria grows wary of the new Swedish land claims, arguing that Austrian traders and trappers had occupied those woods for decades.
Summer, 1390 A.D.: Swedish advances in machinery allow them to begin fielding crossbows. The first units trained to use the innovative weapons are those guarding the Northern Stormlands from the threat of Celtic raids.
Swedish forces meet up with Austrian and Danish camps already besieging Salzburg. The generals of the three nations begin outlining plans to overwhelm the German defenses and end the war with minimal losses.
Winter, 1390 A.D.: Awesomeness becomes the majority religion in Turku and the surrounding frontier settlements.
Gathering that they have learned all they can, the Swedish spy network leaves Edinburgh and returns to Stockholm, setting up to prevent the actions of foreign spies in the capital.
Austria unexpectedly signs a peace treaty with Germany and pulls their troops from Salzburg. The remaining attackers are left baffled, but continue the siege along with Greek allies freshly arrived from the South.
1395 A.D.: Greece declares war on the Celts, somewhat unexpectedly. Sweden, having committed much of its strength to fighting Germany, does not follow suit.
Well, I didn't see that coming. It's good for me in any case, though. Whatever plans the Celts had to attack me are now likely done for, as they'll have to contend with the overwhelming might of Greece.