Sweden was unlucky and that is the reason for pacifism. We all want to make our countries out to be the beacon of the world but truth is the reason Sweden became so peaceful is because they lost.

Now that is not to say that is the reason they maintain their peaceful demeanor but it the reason for it. When Gustavus Adolphus and his successors began their conquests Sweden was recognized as a great power in Europe, an impressive feat for a country with such a small population. It was largely due to Gustavus' military innovations like his mobile artillery and professionalization of the army.

During and after his reign for about a century Sweden was the biggest power around the Baltic but after beating the Polish-Lithuanian Commonwealth it fought the Russian Empire who'm they had beaten quite recently under Charles X but after a crushing loss in the Battle of Poltava.

The Swedes were powerful not because of numbers but their sophisticated tactics and their professional military but once the Russians modernized their military the larger population and the wealth of Russia beat Sweden. It was all down hill from there forcing Sweden to cede most of its conquests out of Scandinavia over time.

During Napoleon's wars Sweden joined with the Coalition and after the Battle of Leipzig it forced Denmark, France's ally to cede Norway. Which rejected to the trade and Sweden conquered Norway as a result in 27th of July 1814, the last time Sweden went to war.

tldr: Sweden was militaristic but after losing to the Russians and losing it's military edge rather than fight large powers they avoided wars unless they could win.

You might be interested in Christine Agius' The Social Construction of Swedish Neutrality: Challenges to Swedish Identity and Sovereignty